What Causes A Heavy Menstrual Period?

I was about a month late getting my period and now I have been on my period for just over two weeks. The first week was light spotting and then it went to a normal flow. But it now is getting heavier. What is wrong with me? I am only 21 and I took a pregnancy test a few weeks berfor I started and it was negative. My cycle runs about every 60 days.