Vaginal bleeding and painful urination

I get frequent UTI's, always have. But I'm concerned because I had sex two nights ago (with a man who shouldn't be able to have children) and I started bleeding today. 11 days before my estimated start date. UTI's have never messed up my period to that extent. Painful urination, frequent urination and vaginal spotting. Is it possible I have an STI or STD? I have some antibiotics sitting around because like I said I get UTI's often. Should I start the antibiotics and see how I feel? Or do I need to get to the doctor asap?