It sounds like your uterine muscles are just a little overreactive. Sex causes your uterus to contract and I read about this a long time ago so I may be off here and there, but the muscles produce calcium or something along those lines which cause them to contract more and it creates a positive feedback loop, like a domino effect. So it makes sense that once you have been contracting those muscles, they may not stop doing that immediately, and may even start to contract more strongly even after an orgasm.
I'm just thinking out loud with that. If they get really bad or it's brand new, I would tell your doctor and see what they say.