Sex before my period.

Help Ladies!! So i had unprotected sex with my bf Yesturday ( i know dumb 😩) and according to my period log i should begin my period tomorrow. I was wondering if i should buy the morning after pill😟 (Why do they make em so expensive 😡)   recently i stopped taking my birth control pills cause my stupid health insurance doesn't cover them and they put me on some other stuff that was making me sick 😷.
  Anyway I mean how fertil can i be when my period is due tomorrow. And what if my period is not on schedule. My app is set at pregnancy prevention and According to my app im at a 2.8%risk whatever that means. 
Thank you for your opinions ladies