Well, when you weren't sexually active but still had an irrational fear of pregnancy is a problem. Now that you're sexually active and relying on a condom for protection, that's not overreacting - condoms aren't that reliable. Can you use a more reliable form of birth control?
I think all women feel this panic at some point or the other. Whether you could be actually pregnant or not, only a test can tell. In the meantime, do your best to avoid an unwanted pregnancy and you can avoid getting stressed every month.
Have you thought of using an IUD, or getting depo shots or birth control pills etc.?
Next, you may want to control your anxiety, and the first step to do this would be to get a reliable method of birth control. If it eases your kind, use a condom AS AN ADJUNCT to another contraceptive method.