Gaining

You must "tossing and turning" more. My wife and I are pretty hefty and our sheets generally stay on, though the fitted sheet on the mattress does tend to ride up and off the corners. Your sheets could also be too small for your mattress. Mattresses have gotten "thicker" in the last few years.

If you're eating a lot before bed in order to hit that new weight, you might be having more restless sleep all around. I used to try getting down big weight gain shakes right before bed before I heard you tend to gain more that way, but it made sleep so difficult I had to stop.