I had my period about two weeks ago for three days. I had unprotected sex last night and I got my period again that morning even tho I wasn't due. Why?

Lots of causes. The bleeding in the middle of your cycle, probably was not a normal menstrual period. Some women bleed at the time of ovulation. If you bleed after sex, that can be a problem and you will want to have an exam for infection, trauma, polyps or other possible causes. Are you trying to get pregnant? Sounds like you had sex at the most fertile time of the cycle.