I took plan b 3 hours after I had unprotected sex. I got my period on time after that but it lasted 13 days and its normally 5. I haven't gotten my period since and I am over 2 weeks late. I took a pregnancy test but it came out negative. Is this a normal thing to happen to someone after taking plan b?

Normal? Not sure there is a real normal since it can effect differently. But it can have effect on your cycle. So yes most likely this is the cause.

Any doubt you should go to your doctor.