Am I Pregnant? Or Just Paranoid?

Hi I'm sorry if this is in the wrong section, as I do not know where this would fit under.. So long story short, the last time that I had sex was with my ex boyfriend on July 26. A month before that I had stopped using the pill, and when me and my ex had sex we used a condom. When August 24th came around, my period came right on time, though it was a little bit lighter than usual, and ended a day early. Then September 24th rolled around, no period. I took 3 home pregnancy tests... All of them negative. I went to the doctor's, they took a urine test -- Negative. A blood test -- negative. Then I went to a women's specialist, and took a urine test there -- negative, and a blood test -- negative. They also did a vaginal ultra sound, and a regular ultra sound. No baby, although they weren't looking for a baby in particular, because the scan ultra sounds were to looks for cysts. Can someone please tell me.. Am I being paranoid about being pregnant? I really can't afford to be pregnant right now. And I thought the ultra sound would calm me.. But I'm still freaking out. The last time I had sex was July 26th, so medically I would be 11 weeks pregnant if I was pregnant. Would they spot an 11 week pregnancy on an ultra sound? Am I freaking out over nothing?