Development of dominant mass

I just had my yearly screening mammogram and was called back for a diagnostic mammo due to a "development of dominant mass within the far posterior breast." I have had documented breast cysts in the past. There is no suspicious cluster of calcifications. The overall radiologic impression was "developing densities within both breast requiring diagnostic mammography and ultrasound." Of course I am a nervous wreck. Does anyone know what these terms suggest and how serious this is? Thanks for any responses.

[Cleveland Clinic]

Dear einnj, It is difficult to comment more specifically on the findings over the internet without ability to evaluate the films.  They have seen something on the screening mammogram that needs further investigation.  Further evaluation is needed before they can really comment on the seriousness of this finding.