Very strange she would do this. A blood test doesn't lie, unless taken way too early. Which I don't see this being the case here. Could be a cyst of something causing this, which they would see on the ultrasound.
I have had 1 negative blood test. My doctor has me getting an ultrasound Wednesday because she said she believes I am pregnant.
Have you had a blood test done? If not, then if you have that done you'll know for sure if any hCG is present in your blood and if you're pregnant or not.