brown discharge / old blood after painful sex

my bf and i had sex 3 days ago and it hurt. we have sex kind of consistently but for some reason it really hurt this time so we stopped. a day passed and two days after we had sex i pulled down my underwear and there was

a brown substance (looked like old blood from the end or beginning of a period). it’s day 3 since we had sex and now i’ve been having light brown discharge. could this be implantation bleeding / is this a sign i’m pregnant? i’m going to take a test tomorrow morning but i just had my period about a week ago (ended the 6th i believe) and according to my app i’m in my fertile window now. i also took a plan b back in late december (the 23rd) so my cycle has been a little off. any advice / thoughts?? please help!