Shaking weird after sex

Excuse me if this is a dumb question but I legitimately don't know lol.

Well today my boyfriend and I had sex, we rested then went at it again and he finished while I was jacking him. So I was sitting there looking at him after he's done and he starts shaking then stopping then shaking then stopping. I was like wtf are you doing but then our baby started crying so we got distracted and I haven't had a chance to ask him. Sooo does anybody know wtf was that or was he just playing.