Period was 10 days late then...

When it came on the first day it was just blood when I wiped and severe pains in my back stomach and thighs to the point I

Struggled to move.

Day 2 was heavy. I say heavy. I only used one 2 pads. So not THAT heavy. But normal.

Day 3 (today) is light. One drop.

Does anybody know why this might happen?