What Is The Painful Brown Discharge And Lower Back Pains After Having Sex?

I am a fifteen year old girl and my boyfriend and I have sex without protection often. The other day we did and I started feeling pain, then I randomly started bleeding. I have had a dark(sometimes light) brown substance coming out of my vagina since then. I dont know if it;s dried blood though because it looks nothing like that. It's really thin,for the most part and i dont think it's blood. also, i'm having MAJOR lower back pains. Could this possibly be a sign of pregnancy? or an infection? Because I really believe it not to be blood. Please help!