The uterus will contract with orgasm, maybe you have a something in the uterus like a fibroid that otherwise doesn't cause you any problems but is bothered by the uterus contracting. Or even some scar tissue from childbirth. Try taking Advil before sex, but also see your doc and ask for an ultrasound or an MRI of the uterus to make sure about whether there is a fibroid or scar tissue.
Did they tie your tubes or did they insert coils / Essure? There have been a lot of problems reported from Essure including migration and nickel allergies / sensitivities.