Involuntary muscle movement!?

Every time me and my partner do the do and he's on the bottom his stomach muscles contract so hard its to the point I have to ask him if he's alright and we have to stop ,he always tells me yeah and that he wants to keep going ,but it scares me because it happens more than once and can happen one after another if I go to fast or move with too much force does anybody have an idea why this is happening at first when we first started having sex I was usually on the bottom which was the position I naturally gravitated to because he's a much bigger man than I am and i like being on the bottom; for me its easier all I have to do is relax and let them learn my rhythm .but as time went on he said that it wasn't fair and that he wanted to switch now its like that's all he wants to doWell anyway can anybody explain the contractions and if I'm doing something wrong how do I correct it