Is Overactive Gland In Breast Dangerous?

My daughter (37) has had a lump in her breast for the past 2 months. It tends to grow towards the start of her menstrual cycle; then diminishes somewhat. She recently had a mammogram. The clinic says all is ok but to make an appointment with her gyneacologist; they say there is no rush.
The first inclination our GP had was that it might be an overactive gland. Can this be dangerous?