Hi Bath, my girlfriend had a similar experience with one of her mammograms. She spent a week being scared out of her mind before the followup test. But it turned out to be just scar tissue left over from an accident she was in when she was a teenager. All signs of the accident were long gone, but the scar tissue was still there.
Surprisingly, she and I figured out what it was before the final test came back, simply by talking about what it "might" be and knowing about her history, something the Dr failed to even ask about.
I blame the Dr for her week of fear due to his callous way of assessing the first mammogram. He could have said, "80% of unknown shadows on mammograms are nothing to worry about, and oh by the way, tiny bits of scar tissue from years ago would look pretty much like this, but let's double check." Instead he simply said, "shadows like this are abnormal, we need you back for more tests immediately."
We did enough google research in a week to fill a library, and we quickly realized it was way too early to get scared, but it was a tough week anyway, even knowing that probability was on her side.
Anyway, good luck.