When is the best time to get tested after having sex?

I am 15 years old and yesterday I had sex with a guy. I am not dating him he is only a friend and I hope I didn't get pregnant because my parents could have him arrested (he is 18). I am dur for my period tomorrow but my period is nor normal at all it comes when it wants because one time I went three months without my period no way I could have been pregnant and then it came. I am worried because he didn't use a condom and he prec*m in me. My family is going to hate me if I am pregnant because they don't like whites dating or having kids with blacks and I am white and he is black. When is the best time to get a test? Also I can't get a test from the store without my mom knowing and I don't want to ask her because I don't want to freak her out would it be ok for me to ask the guy I had sex with if he could get me a test if I don't get my period within a month? If yes how should I ask him without freaking him out? Please help me :'(