Can you know your pregnant a week after sex?

I had sex a week ago, he came in me, bc we thought I already was pregnant. I believe I have started my period today. Ive had ALOT of stress fot the last 3 months do to everything. my period is a little less than a month late. and I took a pregnancy test on april 9th or 10th it was positive. Is stress messing everything up? am I pregnant? or not? I'm going to the doctor tuesday.