Just plain ol' benign cyst.
One funny is that the radiologist has the exact same name as my dh (first and last but with one letter difference in the way it is spelt). So after he told me I did not have cancer we chatted for a moment about Irish heritage and he said that we (well dh anyway) are probably related to him from way back in their ancestry.
And then I went on with my day.
Get a mammogram.