What does it mean when they call you back for another diagnostic mammogram?

Several possibilitie: Being called back for a mammogram is due to concern for an abnormality that needs additional "views" to confirm its presence or determine that it is just a grouping of normal "shadows". It does not indicate that a cancer is there, simply that further views are needed, either by mammogram, ultrasound, or both.

Additional mammo: It means there is a possible abnormality present, but additional mammographic views are necessary to determine whether there is in fact an abnormality, or to better characterize an abnormality. Many times the additional mammo views show that there is nothing to worry about.