Plan b help??

Didn’t even really know what category to put this under but I had unprotected sex last Tuesday so almost a week ago and I took plan b like an hour or two afterwards and everything has been fine but now I’m having something that’s like a period except my period isn’t due for another 18 days and I don’t have cramping or anything its just a significant amount of blood to the point I have to use a tampon.. could this be the plan b? Has anyone else experienced this?