I would have said the same thing as Pam. Blighted Ovum is unfortunately fairly common. I really think you need to work with your OB here. I am so sorry you are going through this and I wish you the best!!!
Hello and welcome to the forum. What did your doctor say after your u/s? To have a blood test indicate a positive result, then the next day an abdominal u/s indicates there is no pregnancy makes me believe you may have experienced a blighted ovum. Do you know what your HCG # is?