plan B & period??

Mikala
I had a period about 2 weeks ago (Feb. 28-March 3), took Plan B one week ago (March 10), and started ANOTHER period on Thursday (March 16). 
This period is super super heavy. My normal periods are pretty heavy, but this is very (sorry) clumpy, and I don't usually have such dark red blood. 
Other than that I haven't had any side effects from the Plan B. 
The period thing is SUPER weird because I've been on a 6 week cycle since going off birth control in December 2015. 
Has anyone else had this happen? Could the really close period be from the Plan B, or is it probably something else?