sounds like clots, heavier blood flow will do that and if its thick it tends to look like 'skin" What i would do is keep a check on it. But if you start noticing you get light headed etc. see a gyno =D
More than likely just a heavy period, you are noticing it because you took the plan b and now you are thinking about it. Stop freaking yourself out. You're ok. It's ok.
If you are that concerned go to your doctor. That's the best advice anyway. And relax, stress does funny things to your cycle.
It may just be clots? When I used to have a period they were heavy and I would often get jelly like clumps of blood that would pass... Is this what you are talking about?