What Causes Delayed Periods?

my last period was late and it started on Dec. 2 but was supposed to start on Nov. 28. It was light at first then heavy with bloodclots. I took a pregnancy test on Dec. 1 and it read negative. Now I am 6 days late this month. Why would this be happening? The last time i had sex was on the day after my period ending on Nov. 5.