What Causes Light Bleeds Followed A Week Later By Chunky Heavier Bleeds?

Hello, my period started last week (a few days to early...im on birth control), it was so light I barely bled; and it was a dark brown colour. Now almost a week later the flow is alot heavier and its kind of red-brown and it looks chunky but not big chunks, little like sugar. Whats going on with my body????