Is there still a chance?

Okay, so, I took plan b and I'm assuming it messed my cycle up. I had a period that lasted for almost 2 weeks on my last cycle. Well, a few days before my next expected AF, I spotted (it looked somewhat like IB) and now my period is a week late. I've read the the pill can cause you to skip AF as your body readjusts, but to ease my mind I tested and got a strong negative result.