Pain in pelvis?

Hey ladies!

I'm pretty concerned. I have a pain in my pelvis on my left side. I can kind of feel it when I pee, walk and use my abdominal muscles. But when I have sex, it really really hurts, mostly if it's deep penetration (doggy, on top, etc) what could this be? For background I have never had this problem before and I am in a relationship so am only sexual with one man. Thank you!!

Also! My nipples have been pretty sensitive and I haven't had my period in a while, although I have had problems with that in the past. I have taken two pregnancy tests, both negative though.