Can so eone give me some advice please!

Ok so last month as in march i had sex unprotected 1 time and it was the day after my period the rest of the month i was on bc pills i messed up a few times on them well now i am due to start my period today as in friday and i have been having light brown bleeding like very very late weds and throughout thursday not even enough to fill up a pad all together ive never had this BEFORE. a period maybe after but never before what can this be?