Nervous, please help

So, my ovulation date was Saturday, and my bf and I had sex unprotected. (We only use withdrawal) All day Sunday and today I have have mild to severe nausea. I read online that some people start having symptoms of pregnancy the day after conception, but I'm not sure if I'm just being paranoid. It's still too soon to take a pregnancy test. What do you guys think? Could I really be pregnant or am I just overthinking it?