I need help!!

I had unprotected sex on spring break and took a plan b the next day literally at the 24 hours. My last period was march 21st , I had no side effect whatso ever right after but on saturday I got home and saw I got "my period" but sunday and today have been brown spottings like when your period is ending. Also I had nausea yesterday and today in the morning but that's gone for now. I've had diarrhea. I don't know has this ever happened to anyone? What should I do?