Hi:
Same thing happened to me. I am a fairly slim girl with severe sleep apnea. I kept telling my sleep therapist that I was not feeling any good effect from my cpap any more. He kept telling me that if i didnt use it, I would feel the difference. After some time of believing him, I went to my sleep doctor and insisted on another sleep study.
It turned out that my machine indeed was not helping me at all. My sleep apea had turned into both obstructive and central sleep apnea and I needed a bipap machine to breathe for me.
Once I got the Bipap, I again felt better. Don't hesitate to see your doctor if you are suddenly not feeling better with your machine.
sarah