To me it sounds like the ligament pain I get. I can't stand up straight and it's enough to bring tears to my eyes. Sometimes I swear something must be wrong in there, it feels like I'm twisting an ovary or the placenta is pulling off, but it's the ligament pain, just like I had with the previous pregnancy. I doubt it's BH contrax or pain from them as they usually aren't that painful or noticable when you are only 20 weeks along, especially in your first pregnancy.

I'd definately mention it to your doctor though, I'm just saying what it sounds like to me but always get things like that checked out just to be on the safe side.