I agree w/Margot....Both of those Vets would have been able to tell you if she was pregnant or not!!
This is confusing.
Gestation is about 63 days from a breeding that took.
I don't understand why the Vets said that. They should be able to check her. A good Vet can usually tell by palpating.
They could do an X ray or ultrasound or hormone tests.
Would be unusual for her to be in heat so soon again. Sometimes a little spotting is not unusual, however, if she starts bleeding heavily it could be a heat cycle or complications.
Any green looking stuff? Any odor?
Hope you have considered spaying her. Shelters are full of Pitts and it is really healthier fir the dog.