Everyone has a different response to sexual arousal, climax, and/or orgasm. It's actually pretty common for people to experience muscle spasms or shaking in their pelvic muscles, abdominal muscles, or even legs and arms, because the body is experiencing a whole lot of sensory stimulation in that moment.
I can't diagnose you or anything, but my question for you would be - how do you feel during those convulsions? Do you feel present and aware cognitively? Does anything else hurt or feel weird? Does your body feel out of control, or do you feel like you might get hurt (falling off the bed, etc)? Those are some of the key things that might help you make the decision about whether this is something that might need the help of a medical provider or is just a fun quirk that your body has.
If it's the latter, I want to emphasize that there's nothing weird about that! Every body has its particular way of responding to things dependent on things like anatomy, how we're feeling physically/mentally/emotionally, and what kind of stimuli we're getting. For example, I have a friend who temporarily loses their hearing for about 30 seconds pretty much every time they orgasm, and they've been checked out by a healthcare provider who pretty much just shrugged and said "Tell your partner(s) not to tell you anything important in the half-minute after you orgasm"!
Nothing happens in contradiction to nature, only in contradiction to what we know of it. -Special Agent Dana Katherine Scully