i doubt its my period because i have it irregular

Hi im hopeing someone can relate but here we go i recently had unprotected sex it was 8/24/13 and 5 days later when i used the restroom to pee i wiped and a brownish looking thing came out im not sure if im pregnant or about to start my period but i doubt its my period because i have it irregular i dont have it for like 4 months but its weird cuz inever had that experience before can i possibly be pregnant.. please help i need honest opinions and i think it to soon to get a pregnancy test