What Causes Bleeding Post Anal Intercourse?

About a few days ago my boy friend and i had anal sex. it wasnt my first time doign this with him. but this time after i had gone to the bathroom there was only a little bit of blood. and later on that day there was a little bit more. it stopped. but for the most part now i cant go number 2. and for the past few days my lower back is hurting. it aches right about my but between my hips. what can i do? should i go to the doctors?