Incomplete Mammo Finding

I had a mammogram recently and got the dreaded letter in the mail that I require additional imaging studies.  So I am set up for an Ultrasound and a Diagnostic Mammo early next week.  I called to get the scoop and pick up my report and was told that the findings were incomplete and that it was a Class 0 and that there was nothing in the report.  Grrr what in the heck does that mean?  From what I hear the new digital imaging being used is pulling more people in for rechecks. Is that the case?  Plus I don't understand why if the mammo was incomplete I wasn't asked to go for a second go round at the time of the appt.  The tech was rather snarly and rough so maybe that was a good thing :) The good news is there is a doctor to read it when I go in on Tuesday so I hope to have a question or 3  answered then.  Thanks for the vent.
Lorraine