I Am Usually On A Very Normal Schedule When Having

I am usually on a very normal schedule when having my period. I normally have it for 3-4 days with cramps only on the first day. However, I am currently on day 12 of a very heavy and painful period. I have had cramps and lower back pain for those 12 days along with constipation. I am 21 years old and do not take birth control. What could be causing this?