What Causes Brown Discharge With Blood Stains After Having Unprotected Sex?

Hi,i had unprotected sex 3 days after my period and accidentaly the guy 'came' inside me....i then took an e-pill..now 7 days after am experiencing brown discharge from my vagina and some blood stains.i have back pains,nausea especially in the morning,headaches and abdominal pains....before this i was experiencing increased vaginal discharge......i have a 21 days menstrual cycle and i was worried if i might be pregnant...or is it some kind of infection?