If I had sex on my ovulation day almost two weeks ago would I be able to test for pregnancy?

I had sex on the 17th/18th of this month, if I got pregnant from those days would I be able to test? I started feeling nauseous and my breasts feel tender my period should be starting in 4 days but if I am pregnant would that make me 2weeks pregnant? Or it doesn’t count? I’m having trouble understanding when the weeks of pregnancy start.