Extremely Heavy and Prolonged Period?

Hi. I'm 18 and just recently started taking the birth control pill (about 3 days before my last period started). My period went on for 18 days, stopped yesterday (when o started the placebo pills section of my pack) and then came back again heavier than ever before today. I don't have cramps, headaches, or anything swollen or tender but my period has never lasted this long or been this heavy before. What is happening to