What Causes Vaginal Bleeding After Taking Plan B Followed By Sex?

So I took the plan b pill 24 hours after having unprotected sex. And some of it dissolved in my mouth and some of it got stuck in my gum but I kept drinking and i think it went down. Few hours of taking it, I had some bleeding. I had little spotting, does that mean I got the whole dosage? That it s effective? Or should I be worried?