What Causes Missed Periods With HPT Negative?

My period was supposed to come December 15, 2014 but it didn't. I have taken anywhere from 10 to 15 at home pregnancy tests and I went to the health department. Every test has came back negative for being pregnant. I still have yet to have a period. My last menstrual cycle was November 17 and it lasted for about four days. My last few of periods have been very weird, One ended and then came back for a whole week like two days later and the other ones have all been very chunky almost to the point that I thought I had a miscarriage.. What could be my problem