What Can Cause Prolonged Light Bleeding ?

hi, i have been having light bleeding for last 20 days(typically 1-2 times a day, a small clot each time). my periods are usually very regular and normal. this problem began after i had unprotected sex with my husband. there was a stain in the morning. i took the emergency pill. however, since that day, this problem has persisted. i was supposed to have my periods from 18th, that too went by in this problem