Dumb question

So i had sex with this guy earlier today. Hours ago. Dumb question:Why am i getting cramps hours later? I never had that before, so I’m confused. According to this app it says I’m on my first day of the fertility window. But we had protective sex. I watched him put the condom on and i saw cum in it afterwards lol. We had sex last month and i didn’t cramp (i just was sore bc it was first time having sex in 2.5 years) i had my period august 4 to august 8th. Is this normal? I’m just little worried