Started my period on 12/31, had sex on 1/9, but on 1/14 i had brown blood in my underwear but it wasn't a constant flow what's going on, am i pregnant?

Ovulation: In a 28 day cycle this would be the time of ovulation and some spotting/bleeding can occur then because of the hormone changes that occur. If your cycle was shorter, it is possible (but not likely) that this could have been implantation. If your period is late, check a pregnancy test and, if it is negative and your period still hasn't started, repeat it in 3-5 days. Hope this helps.