Unusual vaginal bleeding

My period was normal and ended as usual it was a little late but nothing alarming. 4 days after it ended I started getting spotting pinkish in color. Then it started getting progressively worse with some clots. I didn't have any cramping like I do on my regular periods. It lasted for like 4 days then it went away. My periods are usually normal. What could cause this. I took a pregnancy test it was negative so it can't be that.