Please need advise or opinions!!:(

AF is going on about two weeks late now. Saturday I'm gonna try to go to a pregnancy clinic to find out for sure because all of my home tests have been negative and I've taken 3 or 4 of them. I'm terrified. I'm always nauseous now, always feel like I wanna puke, my nipples are sore....as of right now it would have been 3-4 weeks since unprotected sex, yes he came in me. I'm on the pill but also on other medication because of medical problems. I'm seriously scared because I always feel nauseous recently. Any ideas on why AF may be late other then pregnancy would be great! Thanks!