I Had My Period Starting Feb 5...had Sex With My

I had my period starting feb 5...had sex with my bf on 20 th feb .i used a condom..got no periods aftr that...a gynaec sent me for ultra sound scan on march 22...according to report, im not pregnant....she told me to wait a week and prescribed deviry pills on apr 3...i dont hav othr pregnancy symptoms...i m a woman with regular periods of 30-35 day cycle , 21 yr old...could it be that the scanning report was wrong ? I m worried..pls help