Oh gosh - I just started experiencing what you described this afternoon. The SAME exact thing you described. I am 19 weeks pregnant when I stand up it feels like there is something heavy pressing down from the inside. Then it's achy when I walk or lift up my legs. It seems like the 'pain' (it doesn't really *hurt*) goes away after a bit, but I can still feel the pressure.
I know for a fact that I am carrying very low. Hopefully that is all this can be attributed to ... along with the normal feelings of pregnancy. I don't have any bleeding or irregular discharge. Do you?
I called my doctor's office - but of course they're not taking anymore messages today unless it's an emergency. This is my first baby, and I am not scheduled to go in for my next appointment till Monday. So I will probably call back anyway - emergency or not. I am not thrilled with my OBGYN!