Hi, and thanks for your medical question on eHealth forum!
If the inverted/retracted nipple is present since birth then there is a genetic reason behind it. Otherwise it can be due to breast feeding, previous pregnancy, previous or current infection and scarring, sagging of breast, tuberculosis, or even breast cancer like Paget’s disease, Inflammatory breast cancer etc. Since mammography is normal, cancer is ruled out. There are 3 grades of inverted nipple.
In the first grade, the nipple can be easily pulled out and grade 3 is a severely retracted nipple. Grade 2 is in between. Please consult a doctor and get this investigated. Grade one nipples can be pulled out during breast-feeding. Retracted nipples of Grade 2 and 3 types need surgical correction usually. Take care!