help i can't be pregnant

i'm 16 and really hope it not pregnant. i may just be over reacting. i am new to the app and my cycle on the app might be a little off so idk. my period usually comes around the 20th of each month and i usually experience low back pain. but whenever my boyfriend and i have sex we always use a condom and it has never broken and he pulls out before. i'm super worried because i woke up this morning with brown discharge, my stomach hurts and i feel like i'm gonna throw up, and i have diarrhea. i took a plan b pill too about 4 days ago. i'm so scared someone please help!!!!