Prevent butt cramps/side cramps during masturbation

So, I'm still new to the whole masturbation thing and I'm exploring.

I notice that at times I get cramps on my left side or my butt. Which keeps me from having fun 🙄

I just lay down on my back and my legs are wide open on my bed. I've tried using an extra pillow for cushion but that doesn't seem to be helpful.