Hi. I've Had A Pretty Regular Period For As Long

Hi. I've had a pretty regular period for as long as I can remember. I had my last regular period the last week of November. On December 11, I started bleeding again, which is not normal for me. After about a week it slowed down & was like spotting for a few days. On Tuesday, the flow became very heavy & with large, gumball sized clots. I've had mild cramping but that's about it. The color is normal. I'm 21, & ive been changing my pad every 1-2 hours over the past 3 days. What is most likely causing this?