What Causes Excessive Bleeding After Taking Pregnancy Pills?

I am 19 years old and sexually active. This month, my menstrual cycle began very slight and I wasn't releasing as much blood as usual. On the third day, I started taking pregnancy pills for the first time (as my gyno prescribed). The same night I started bleeding EXTREMELY excessive amounts and it's been going on until today (3 days so far - that's a total of 6 days of my period until now and it usually only lasts for 5 at most).
What's going on? How I can fix it?