Is this my period?

I am supposed to start Ortho tri cyclen this month once I start my period. However, I took a Plan B pill in late December and for the late 4 days I've been having brown discharge and cramps. I thought it would go away and my period would start but now it is getting heavier. My period is supposed to start on Saturday. I am having random spurts of a lot of brown discharge. Should I start the birth control now or should I wait until Saturday? Or should I wait until I actually bleed red like a normal period. I'm just not sure if plan b messed me up this month and I won't get my normal period.