Stomach ache and nausea after sex

So me and my husband are trying to get pregnant. I have been off birth control for 3 month and next Wednesday I should start my period but the last couple times we have had sex this month I get so Nauseas and stomach ache. I took a pregnancy test  today and it said negative but could I may be pregnant or what's going on? Is it a backlash of just getting off birth control and PMS?