Side effect.
Are you sure it couldn't be a period? I've had periods like that before and I didn't even have a reason for it like plan b.

When did you take the pill again? Like how long ago was this?

I'm going to agree with Maddie and say side effect. PlanB does AWFUL things to your system depending on your sensitivity. My first time was before I was on any birth control and I had insane cramps, threw up, got a really early and light period (which it sounds like you're getting). Also, so you know, implantation bleeding is rather rare. Most women do not experience it so it's not safe to assume that you're pregnant because you have what looks to be a light early period. If you're really nervous you can always take a pregnancy test 14 days after sex and it should be accurate. Good luck!

i got a light brown discharge one week after taking an overdose of plan b then got my period 5 days later.