What Causes Abnormal Vaginal Bleeding?

I was supposed to start my period on the 28th but instead I had started this light bleeding a week earlier than my period was supposed to start. It lasted for about 5 days. After it was over, I continued to see a very light pink discharge. Then on the 28th I started bleeding again but like my normal period. It usually lasts for 5 days but this one is now going on to its 7th. I have not had sex in a few months and do not take any birth control. I've taken morning after pills but that has been a while as well. What would be the cause of this abnormal bleeding?