That happened to me last year around this time, and I to knew there was no way I was preggers because hubby has vasectomy. Anyhow, it turned out to be an intestinal infection which landed me in the hospital on X-mas. I did not have stomach pain, but I began feeling really run down, nauseated in the morning, etc., and then it progressed into back pain, and then it progressed into back pain that felt like back labor, which is why I went to the E.R>...I thought I had a kidney infection as did the doctors until the did a CT scan to rule out kidney stonesor appendicitis, which I also did not have. Turned out to be diverticulitis which is rare in younger adults, but even rarer for it to present with my symptoms. Anyhow, I would give your doctor a ring, they can do a CBC to rule out anty sort of infection/illness. Feel better soon.