Plan B side effects? If you’re going to be rude just keep scrolling.

My husband thinks he had a little oops in the shower so we got Plan B the very next morning just in case (don’t judge 😓) we have 4 month old twins right now. I’ve taken Plan B one other time several years ago and never had any issues with it or side effects. My cycle literally just ended on Sunday morning and I took the pill Monday...and last night I started lightly bleeding and having cramps like I’m on my cycle again? (Except my actual cycles are very very heavy since given birth) what could this be? Side effects from the pill?