If you have been to the doctor and they tell you you are not pregnant chances are you are not. If there was any doubt in their minds then they would do an ultrasound. But if not already, you can ask them to do a blood test, more accurate than urine.
Have you ever heard of a hysterical pregnancy? You should look in to it. There have been cases of women that were so certain they were pregnant that all the symptoms were there, weight gain, pains... etc... but no baby. It's interesting. Not saying you are going through that but once you of peace of mind that you are not pregnant than maybe the symptoms won't appear to you anymore.