why is my period late

i had intercourse but he was wearing a condom and didn’t finish and i got some spotting a day after for two days and my period was supposedly supposed to start last week but it still hasn’t.. and i know for a fact i messed up my period bc i missed some birth control pills and i’m freaking out a little.. why is my period so late.. i also just started college and i have been walking a lot and exercising more often than usual- does exercise delay your period?? and i have been stressed so i know that can impact it but is my period late because of college and how i’m now going through a completely different reunite?? please lmk!!