Bleeding But Not Time for My Period

Hey Guys,

I need some help. I'm going to the doctor but still I'm curious. I have been bleeding since yesterday although my period isn't due until like May 25th. At first I thought it might be an early miscarriage but I took a Plan B pill on last Monday, May 2nd so I'm wondering if that may be why. I also took a pregnancy test today and it was negative.

What do you guys think is going on?