Period again after 2 weeks of being off.

I've been off my period for 2 weeks now, yesterday I experienced a random bleeding, which was the first time that this had ever happened.
I've always been irregular so this came as a surprise.
​I took plan b about 1 week ago, which I doubt has anything to do with this because it's never happened before.
Does anyone know what can possibly be going on? Help? Ideas? Anything.