Hello, my last period was August 16th, cycle length 24-25 days, ovulat…

Hello, my last period was August 16th, cycle length 24-25 days, ovulation was probably August 26th, since September 5th the tests were positive, I expected menstruation on September 9-10th. Today is the 30th day of the cycle, I had an ultrasound, endometrium 11mm, a fetal egg is not visible. The doctor did not see pregnancy during the examination. All these days my lower abdomen hurts and sometimes I feel nauseous. If there is no pregnancy, why are all the tests positive?