Probably not serious
Most women would have a reflex reaction to any changes in the breast.
...Cancer. Malignant cells occur all the time in the body, but are destroyed by one of the many bodily defenses. When they grow out of control and become 'stronger' than one's native cells (years) cancer develops: painlessly--not tender/hurt. Probably hormonal, less likely early infection. As always, persistent sx--see the doc.