How long?

Lorraine
My boyfriend pulled out but told me it was really close and that maybe a bit of his sperm got inside me. This was about a week and a couple of days ago and I just got my period three days ago. I was wondering if I could still get pregnant. I know I got my period but I've been told of cases where the mother still has her menstrual cycle while being pregnant. Also, I rarely have my period. Maybe three or four times a year, so yeah, don't know if I should get my hopes up or if I should worry. I'd really like to know how long I should wait before I can take a pregnancy test, or if I shouldn't even have to worry about one.