Stress can delay your period, or make you miss it entirely for a month. Also, home pregnancy tests are very accurate, and the probability of getting four false negative results in a row is lower than anyone would care to imagine (unless it were an ectopic pregnancy, which would be noted with bad unilateral pelvic pain; I wouldn't consider this a possibility in your case). If you're very scared of being pregnant, your body is probably playing mean tricks on you, making you think you're pregnant, hence the symptoms. Or perhaps you have a stomach bug that's making you sick (this happened to me when I had a pregnancy scare a couple years ago).
So, to answer your question, yes it's possible to have these symptoms without sustaining a pregnancy. These symptoms could all very well be in your head as products of stress.