It apparently has been a while since I changed it. Probably since the last time I did this reminder. Dang, Paula, you is nasty, girl.
So here is your reminder: Time to change the filter!
And while you are at it, check everything for cracks, holes, etc.
How is the mask? Are the pieces parts still holding together?
What about the hoses? When's the last time they all got a really, really good cleaning? And that humidifier tub, how is it doing?
Give the entire machine a good once over, wipe it all down, clean under it, etc. Tell it you love it very much and appreciate all it does for you.
Take a deep breath and count to zen.