I got off my period like 2 weeks ago. I just had sex with my fiancé, went to pee and a blood clot came out. Past couple days my nipples have been sore and felt sick all day. What could be the cause of this!!

There are many possibilities. It could be he was too rough when you had sex, it could be an infection, it could be pregnancy, it could be cancer, it could be an STD, etc. etc. etc. There's no way to diagnose online.

If you feel it may be pregnancy, wait until your period is at least two weeks late, then test using first morning urine. Me, I'd get checked out by a doctor, since passing blood clots isn't a sign of pregnancy.