I wish I had a quarter for every time I had to go back, and so far everything has come out ok, had a few biopsys and a ciouple lumpectomies, but I have fibrocystic breast disease. I would not worry much yet.
It's not all that unusual to be called back for additional views after a mammogram. Evidently there is an area that the Radiologist saw an area that he/she felt needed to be magnified for a better view. This could be something as simple as breast tissue that is folded over on itself creating a shadow from one view ... I personally have a friend that gets a call back nearly every time. It's good medical practice ... they are being vigilant so nothing is missed. Also as we age breast tissue becomes more dense and often needs more than one or two views for adequate examination. Regards ...