Had sex 3 days in a row with bf- was fine until 3rd day- was enjoyable but a little painful, today i have mild pelvic pain. what could this be?

Pelvic pain: Might just be "too much of a good thing." you may have developed some mild strain/sprain of your vaginal muscles form the 3 days of sex, especially if any of the sex was a little rough, etc. Best wishes.

UTI: It is very common with recurring or frequent sexual activity to get a bladder infection or a cervical irritation and infection.