Very light vaginal bleeding

Okay so I had unprotected sex about 3 months ago and today I have been feeling tight in the stomach and for some odd reason very light vaginal spotting.. I don't think I'm pregnant as I took a pregnancy test 2 months ago and it came back negative.. Some one help I'm confused and I particularly don't want to visit a doctor.. For personal reasons at home.. Also need to add been having what seems to be my normal period.. I'm very clueless and 20 so I need answers please