Constantly irrationally fearful of being pregnant

Hi, I'd love some help please! Maybe someone else is feeling the same way. I'm always worried that I'm pregnant to the point where I've stopped enjoying sex altogether. I feel bad about that but I mean it's been four months since I last had sex and I've had four normal, on-time periods but I am still convinced... I even took a test today because I couldn't take the anxiety. Of course it was negative. This paranoia / anxiety is driving me crazy I can't concentrate on my studies and I've been losing weight (not a thing I need to lose) help :(