When To Take Pregnancy Test?

I have a question about sex. I am 18 years old, 5'8", I weigh 143lbs and I don't have any medical history except irregular periods. I had sex last night with my boyfriend while I am on my period. Last night was my 2nd day on my period. How do I know when to buy a pregnancy test? I'm confused. We didn't use any control because we were under the impression that it wasn't possible to get pregnant while I'm on my period. Apparently it is possible because I asked a website about it.