I'm not sure how long you are feeling this sensation for each time, but it sounds like you are possibly experiencing Braxton Hicks? However,BH are not painful when you get them. More or less just a tightening feeling and a bit uncomfortable...
My first pregnant I would get that sensation after I would have an orgasm. I just thought it was the baby moving to one side. I don't think it's anything to be alarmed abou, but just ask your doctor.
Sounds like BH to me too. And the further along your gonna feel the tightening when you orgasm. Sounds pretty normal but you might want to mention it at your next ob appt.