Feeling weird

Every time a couples days  after me and my bf have sex. I start to feel weird. Then I start feeling weird like I might be getting pregnant symptoms. But every time I tell him I feel nauseous he tells me to take the test. And when I do it says not pregnant. But this time when we had sex was different. I feel nauseous every morning but when I talk to him he doesn't believe me. He is like oh it could be that ur just stressed out.  Help me I don't know what to do