Should I be spotting?

So today (monday) is my third day period free (since friday) and I had taken a plan b before it started and my period was 6 days late. This was my longest period yet, theyre usually 3-4 days & today my boyfriend was about to give me head & when he pulled off my panties i noticed dark red/brown stains?? It's been happening since my period ended and usually its only the day after so im wondering if its because of the plan b? Or is something off?