I have the same thing, I am 25 years old, my former gyno felt a lump and ordered a bilateral ultrasound last year, nothing abnormal showed on my ultrasound. This year I went to a new gynocologist who informed me that the lump was most likely from the weight of my breast pressing down, causing the tissue to stretch and scar over time.
Maybe, sometime in your past, you had an injury... like a hard blow to the breast.
And, it left a knot which, in a mammogram, might look like scar tissue build up.
Anyway, that's just my guess.