Dark brown period

Hi! This month for the first time (I'm 22) I got a very dark brown period, almost black.

It lasted only 4 days, on the first day I had cramps. Usually mine is very red... It also came late (23 instead of 10)... It ended few days ago, but I still have pain... Is it normal? Could be implantation? I had only safe sex..however I'm a little bit worried