It's definitely possible to have a false negative, b/c in order to test positive, your HCG level has to be high enough to trigger the "+" for your pregnancy test. Most tests in the OB's office have a really high HCG level, so it's common to get a false negative on those tests. I recommend either waiting a few days (if you can take it, LOL!) or specifically looking for a pregnancy test that will test positive with a low level of HCG, like 25 or so. You can also request a blood pregnancy test, but again -- unless your HCG is above a 5, it will come out negative. During your first few weeks of pregnancy, your HCG level will double every 48-72 hours, and once that level is high enough, you can get a positive.
Don't worry about the stress -- everyone tells you not to stress out, and if it were that easy then there would be no stress in the world, LOL. I highly recommend reading the book "Taking Charge of Your Fertility" which will explain to you exactly how to time sex to have the best chances of pregnancy. I think every woman should read the book -- concieving or not -- b/c it's just fascinating to learn how your body works.
Also, for some good information about pregnancy tests, I love the website www.peeonastick.com. It's very humorous, and explains all about HCG levels and whatnot.
Finally, good luck to you! I hope you get a positive test next time you pee on a stick! :)