What Causes Swelling Above The Pubic Bone After Having Sex?

I am a 24 year old female, after masturbation (clitorical), I felt swelling and tightness in my uterus; almost like there is a small hard balloon sitting right above my pubic bone. It's been a few weeks and it does not go away; and it the symptoms come back even stronger during and after sex to the point that I cannot even lie on my stomach because of the pressure. And my bladder is full, it seems that my belly swells as well.
There is no problem with my period, but I have noticed a bit of constipation (perhaps unrelated?). I have taken a break from sex, and it seems to be improving a bit, but I am really worried it might be something serious. I was thinking that this is a muscular problem, like overstressed muscles that have cramped, but it just doesn't go away. Please help!