HELP!!!! UNPROTECTED SEX

so I had sex a night and a half ago. We didn't use protection and I'm supposed to get my period in 2days (I'm irregular btw). As of yesterday I've started feeling nauseous and tired. I bought a morning after pill that is only effective within 3days after unprotected sex. I also got a small red stain in my underwear but not sure if it's because I'm getting my period or signs of pregnancy. Am I pregnant?/ Can one start getting symptoms after the 1st night ? HELP PLEASEE!!!