False positive test

I was 4 days late on 11 May, so I took pregnancy test it was positive and I was 5 week pregnant also I do not have any pregnancy symptoms. Today morning 14may I started bleeding so I rushed to ER. DR and Nurses checked me well including my pelvic examination, blood test as well as Ultrasound. DR told me he did not see any positive pregnancy things even nothing was in my ultrasound. Doctor fails to explain me what happened to me and said it was not the pregnancy. I don't know if it is common or not. Please help