I just went through this. Sometimes the just need more imaging because they are using different machines that show things differently from the last time you had a mammogram at that facility (of course, that only applies if you used that same facility previously). I get called back every single year for breast ultrasound because I have very cystic breasts that make what they see on mammogram hard to read, so they do an ultrasound to double check the mammogram readings.

This is the first year I went through all of the above (mammogram and ultrasound) and had to go back for a biopsy because they saw a place that needed further investigation. Even then, they told me they weren't too concerned, that they felt pretty sure what they were seeing was not a malignancy, but they were doing the CYA thing and doing the biopsy to make sure. Everything turned out just as they expected.

I would say don't get too worried just being called back for more imaging. Until you reach the point of biopsy, they are really just double checking their readings. Easy to say not to worry; harder to do. I completely realize that. But honestly, try not to get too overly worried quite yet.