Had unprotected sex about a month ago ... he nutted in me but now i keep have pain on my right side right by my rib cage. what could cause this?

Pain in the ribs: Pain in the ribs is in no way related to your sexual history. I am not sure why you included it. Anatomically these are very separate. Rib pain on the right side can be a liver or gall bladder problem; it can be a muscle spasm or a rib or chest injury. It can represent anxiety. I would advise getting in to a physician to examine you and diagnose the problem for you.