Im 19 and i been sexually active for a few months. I have been having safe sex with a condom every time. Last time i had sex ( a week ago) i began to spot right after and its been sometimes heavy, sometimes light, sometimes very dark brown, sometimes light browns, and very bright red heavy or small bright red spots. My period is due soon in a few days But I still took 3 pregnancy test and the all where negative. I have no other signs that can be alarming. just the spotting for a week now. what can it be?