help!

marissa

so I started my period July 5 (had unprotected sex that day plus 2 days after and 1 day before) ended my period July 10 (had sex again unprotected the 15 and 16) I wasn't bleeding at all until the 15 then I started spotting it was red blood but now it's just brown blood. it is now the 18th why am I still spotting/spotting at all ? :( what could be wrong???