I Hade A 4 Day Late Period.(which Im Never Late).

I hade a 4 day late period.(which im never late). I had brown discharge for 2 days. The next day nothing. The day after that I woke up to use the bath room and before I could reach the toilet I felt a gush of blood. It was dark colored and I passed a few tiny bits of tissue nothing big but the night before I had light cramping. What could this be?