confused

I really need some advice. I have a lot of signs and symptoms of be pregnant so my friend and I rented one of them dopplers off line. When we finally got it and test it out we could hear a very fast heart beat of a baby or so we thought. I was in hospital last night and they did blood test that said negative. Is it still possible to be pregnant? What else could make that sound of a baby heart beat inside of me? The hospital didn't do a vaginal sonogram. Please help with any advice. I will maybe be back in different hospital today.