Periods and Birth Control

So within the past month I’ve started birth control. I’m currently taking Sprintec and on my placebo week. I started my period yesterday and it was pretty normal looking, bright red blood, all that stuff. Today I go to use the bathroom and realize that it’s all really really dark brown, almost black looking with a thicker consistency. I did some research and what I’m understanding is that there’s not a lot of blood when you’re on birth control and it’s just other stuff coming out. Is this normal? I’ve only been on birth control a month so i’m still in the adjusting period.