My period after Plan B??

Last night I began to have super super bad cramps with minimal bleeding at the time but the bleeding was a dark bright red. Today the cramps are still bad and the bleeding is deep red/brown and is sorta a light flow. My period was set to start 4 days ago. I took a Plan B 3 wks ago and then basically had a period for 4 days after. So is this my actual period coming on? Normal?