HELP!

Me and my girlfriend had unprotected sex for about 5 minutes on the 28th of April. I put on a condom after those first 5 minutes or so. I did cum inside of her (with a condom. And it didn't break). However, I'm not sure if I pre cummed in her or not, during the first 5 minutes. Her period should be coming on the 2nd of May. And I'm wondering when would be the best time to take a pregnancy test ?