A Couple Days After I Finished My Period, I Had

A couple days after I finished my period, I had unprotected sex. He ejaculated inside me so we immediately bought plan b. I took the pill at 3 am on Saturday morning. Today, I noticed dried blood on my underwear. Not much so I didn't worry but while sitting in my bed, I felt blood pour out of me. Almost a feeling of peeing. My periods are usually very light and only last 3 or 4 days at most so this is kinda scary for me. I've never taken plan b and I'm 18 years old. Is this normal?