You COULD be pregnant, but I was one of those few women who experienced implantation bleeding (most don't) and it was only a teeny spot once a day, miniscule, for like 3 days. Doesnt sound anuthing like yours but I guess it could differ like every pregnancy differs. It sounds like an off period perhaps. Stress (maybe from worrying about pregnancy) can cause an abnormal period sometimes. If you are pregnant though it may be too early to test. Test in a week if it'll make you feel better. If it comes bag negative, breathe, and watch out for your next period. :) Best of luck for whatever outcome you're hoping for!
Plan b doesnt always work, you could still be pregnant. Also, plan b messes up your cycle sometimes, you might not be pregnant. Keep testing for pregnancy until you either get a positive test or a regular period.
I guess there is a good chance you could be pregnant, I've heard some girls still got pregnant after taking the MAP. I would wait a week or two an take another test just to be sure. don't stress thing will work out, everything happens for a reason!! Good luck