What Causes Dark Vaginal Spotting Post Unprotected Sex?

Ok I had sex with a guy Dec 12 he let his sperm out in me a lot it had been a while sense we had before then I had a really dark spotted period for a day it went away I had sex with abother guy he never came in me on Dec 23 the one that came in me we had sex again on Jan 3 and he came in me again on the 10th I had light implantation bleeding who's baby is it