Odds are it is nothing. Some medications can change hormone levels (or even just your body itself), leading to higher prolactin levels, meaning milk ducts get more prominent. I was around 32 when my doc started sending me for ultrasounds & mammograms. First it was the OB/GYN and then the PCP, once they started doing well woman exams at my PCP's office (so long as you weren't pregnant or needing more than a routine pap smear or birth control pills).

Try not to worry until there is something to actually worry about. Easier said than done I know. One of my grandmother's (now deceased, but from Parkinson's) had a grape-like cluster lump in her breast when I was in high school. The thought it was surely cancer, couldn't tell from the US and mammogram, but the biopsy came back benign. She lived another 20 years or so, like I said, it was Parkinson's in the end.