difference in size from mammogram to ultra sound

A week ago I had my annual exam. Three days later I recieved a certified letter saying I needed further follow up. I already had an appointment with Dr. for Monday, when I asked about the results she had no idea what I was talking about and went to find the report. She agreed that I needed an u/s "very small" calcification about .9mm. Appearently it has been in the previous 2 exams but had changed. Had u/s this morning, radiologist says it is a cyst about pea sized. Could the mammogram missed the cyst or ignored it and the radiologist missed the calcification? I ask because of the significant difference in sizes. We live in a rural area that only has limited visits from some specialists so everything goes thru the GP first. No family history. Any input would be helpful.

Good News! Found out not a problem with the radiologist. My GP can't tell the difference between .9mm ("smaller than a pencil lead") and 9mm. It is a simple cyst and benign fibrocystic condition.