Helllo! Sounds like you might be pushing into the body of the guitar with your right forearm / elbow. Try resting your arm more lightly on the top corner of the guitar body instead of pushing inward and squeezing. The guitar should be in a stable but relaxed and balanced position on your lap regardless of what the left hand is doing.
Let us know if that helps!
-Carl.
Carl King[br]GuitarTricks Video Director / Producer