Not viable pregnancy? Thoughts!

I have had two miscarriages, so when I found out I was pregnant, I wanted to get ultrasounds as soon as I could. I had an ultrasound at 5 weeks and there was a tiny speck. Then again at 6 weeks and it was blinking and the doctor said that was the heart beat. However, it took her a while to find the sac and the baby. I went in for my 8 week check up and I had a different doctor. He couldn't find the uterus. He found a sac that he said was my uterus and said there was no baby. I haven't been bleeding or anything... I think he's wrong! Where did the baby go? Can it just disappear???