Why do I have dark brown discharge?

I've been having dark brown discharge of a couple days now. I've never had this before. Could it be because I've took the plan b pill 3 times since January? One in January, one in march, and one in april? There is no pain or itching it's just dark brown discharge. When it first happened, it was kind of pink, and then it turned light brown, and now it's really dark brown. It looks like poo but it's not cause it's coming out from my . Just giving a clear imagery of how dark brown it is... Any answers can help ASAP. Thank you so much