Can this cause my fatigue?

[quiescence at last]

Also - try a rolled up pair of soft socks (I use 3 folded together) to fill the space between the pillow and your cheek, so the mask is not shifted by the pillow or your arm.

[quiescence at last]

You're machine does not have EPR. Instead the technology this company uses has two components. (1) pressure relief equal to 2 cmH2O (quite similar to an EPR of 2) and (2) a Flex setting of 0, 1, 2, or 3. The flex setting exaggerates the pressure relief by a certain amount per breath dependent on the inhale volume. With flex setting on you get a variable extra pressure relief that changes every breath.

With AFlex setting of 3, I found that I just felt like I was being forced to exhale when I did not feel ready. It was irritating and not at all conducive to me getting to sleep. I alternate between Aflex=2 and Aflex=1 for comfort. I am currently using Aflex of 1 and I can hardly tell the difference between 0 and 1, and my exhales do not feel forced or badly timed.

Good luck.

QAL

[Brazen]

Full night of treatment only here or there is not enough to feel improved, so it makes sense that you still feel crappy on those days.
Think of your current state as living in sleep debt... significantly in the negative.  It can take many nights of optimized treatment to feel better.
The lucky folks feel better right away. The rest of us have to work at settings, mask fit/leaks, overall comfort, and sleep hygiene. 

About side sleep...
I can only sleep on my left side. I manage to do that with a full face mask and little/no leak.
Masks are incredibly individual. And take way too much expensive trial and error for proper fit and strap adjustment. 
I put the side of my face at the edge of my pillow... like ear on pillow but mouth/nose/mask off the edge. Prevents the pressure of the pillow moving the mask out of alignment.  
I put my arm up in front of my face.

You don't have to force yourself to sleep in a way that isn't comfortable. Find a mask that works the way you want to sleep.

[The Friendly Zombie]

You're machine does not have EPR.  Instead the technology this company uses has two components. (1) pressure relief equal to 2 cmH2O (quite similar to an EPR of 2) and (2) a Flex setting of 0, 1, 2, or 3.  The flex setting exaggerates the pressure relief by a certain amount per breath dependent on the inhale volume.  With flex setting on you get a variable extra pressure relief that changes every breath.

With AFlex setting of 3, I found that I just felt like I was being forced to exhale when I did not feel ready. It was irritating and not at all conducive to me getting to sleep.  I alternate between Aflex=2 and Aflex=1 for comfort. I am currently using Aflex of 1 and I can hardly tell the difference between 0 and 1, and my exhales do not feel forced or badly timed.

Good luck.

QAL

That's creative advise with socks, I could go for it if need really be, but I am feeling quite comfortabel with using the mask. Sure, being without it is of course miles better. That mask, it is present and distracting when I go to bed. I can hear myself and I am aware of my own breathing. I sound like Darth Vader. There's also a vibrating noise, but only when I either exhale or inhale, don't remember which one now... but might be from the DreamStation!

But when I wake up, I can barely even feel I the mask on, I have to touch it with my hands in order to confirm it is there hah. 
And it is good to know something about the machiney. I will try Aflex 2, perhaps it will improve the comfort. I'm used to Aflex1. 

Thank you for your advises! 

Full night of treatment only here or there is not enough to feel improved, so it makes sense that you still feel crappy on those days.
Think of your current state as living in sleep debt... significantly in the negative.  It can take many nights of optimized treatment to feel better.
The lucky folks feel better right away. The rest of us have to work at settings, mask fit/leaks, overall comfort, and sleep hygiene. 

About side sleep...
I can only sleep on my left side. I manage to do that with a full face mask and little/no leak.
Masks are incredibly individual. And take way too much expensive trial and error for proper fit and strap adjustment. 
I put the side of my face at the edge of my pillow... like ear on pillow but mouth/nose/mask off the edge. Prevents the pressure of the pillow moving the mask out of alignment.  
I put my arm up in front of my face.

You don't have to force yourself to sleep in a way that isn't comfortable. Find a mask that works the way you want to sleep.

Thanks you!
It's a lot, but could very well be. I used mine faithfully for 3 months the time I received the machine. Since then, I can't really say I have used it consistently. And it is true, that I used mine here and there. A week there. Maybe another two weeks there. But spread. Long breaks.  
We figured out, doctor and I, that I have the autoimmune disease "hypothytoidism" which also causes fatigue. But my blood work is according to them, good and in order now and have been so for a long time. I have been invested in that. Yet I'm still as fatigue, and unrested.  

So I am suspicious about sleep apnea. Also that I simply must have used mine too less to begin with. I have heard mentionings about people where it took them 6 months before they felt an noticeable effect. Maybe it takes even a year for some. You and I sleep the same way, I also lie with my mask on the edge of my pillow.

[Brazen]

I get it. I'm suspicious of sleep apnea as well.  Especially since the medical community is so completely rotten about treatment of the disorder!
They look only at the AHI and hours using the machine. AHI less than 5 and using machine at least 4hrs a night -- doc marks you as fully treating your sleep apnea and sends you on your way.
All it takes is a little thought to know that is only the tip of the sleep apnea treatment ice burg!  What about leaks? What about flow limits? What about respiratory arousals that surely affect sleep quality without being flagged in AHI?  What about sleep quality?!

Sigh.  I dislike the whole thing.
The only thing that keeps me using it is the thought that surely my body can get better rest if it's not struggling to breathe.
Take breathing difficulties out of the equation and maybe I can find a doc willing to look deeper.

Hypothyroidism is known to cause significant fatigue. And again, many docs will look only at a tiny piece of that puzzle, namely TSH level.  
They get your TSH level normal, tell you to keep taking your medicine, and mark you down as fully treated.
What about T3/T4? Free T3 and total T4? Reverse T3? 
Try to find yourself a good endocrinologist if possible. A good one will look at the bigger picture.

[quiescence at last]

I found that consistent nasal breathing without undue flow resistance can only be accomplished (for me) if I elevate nose about 60 degrees above horizontal to allow air flow through both nostrils, because if one or the other is not free, there is too much resistance and results in my awake thinking to shift to mouth breathing. That defeats my intent to be asleep, and rely on nose only breathing for much calmer and consistent sleep.

QAL