What Causes Brown Discharge With Clots?

I'm 27 and have always had relatively consistent cycles. My last period was lighter than normal and 2 weeks later I had brownish spotting. It's day 5 of this now and the spotting has gotten heavier, but not as heavy as normal bleeding. It's not consistent either. Tends to come out in clots type chunks 2 times a day. What could this be?