What Causes Bleeding Within The Week Of Taking Plan B Pill?

I had my period 7 days ago started May 26-29 It was normal my period only last 3 days. On Aug 1st I had unprotected sex and took the plan B pill that same night but that same day I drank beer. Now on Aug 6 Im bleeding again its dark brown, thick, but also kinda watery. Is this normal or can there be a possibility I might be pregnat?