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Inverted nipple can be due to infections,inflammation as duct ectasia,abscess,mastitis,hormonal changes and rare conditions as Carcinoma.
You require a detailed clinical examination which will help in giving a better picture of the state as nipple retraction is graded according to the inversion and then treated accordingly.
Grade 1 and 2 nipples can be pulled manually (under Medical Supervision)but greater degree of inversion require surgery.
Meanwhile apply warm compresses to the breast 3 to 4 times a day.
Wear a good supporting bra as sagging breasts can also be tender and nipples can retract.
Kindly visit your Healthcare Provider for evaluation.
Take Care!