i was going to say the same thing.
This same situation happened to my best friend, and i even told her what Ingi said, there are alot more hormones in your body now because you took plan B while taking birth control, which is why your period is lighter than normal, the extra hormones kind of mess with your body a little bit.
You take birth control religiously you said, which means theres even less of a chance of pregnancy, plus the plan b terminates any pregnancy with in 72hours, and you took yours an hour after. Your not pregnant, just having a weird period issue due to extra hormones in your body.