early pregnancy symptoms?

A month ago I had sex on my placebo pills on the Saturday night (finished my period) on the Tuesday I started feeling nauseous and had frequent urination and so tired. I still have these symptoms. 
Would It have been too early for pregnancy symptoms, 3 days after sex? I just don't get how it works when on the pill because I don't know if you ovulate or not! 
PLEASE HELP! My HPT was negative and I got my period.