Brief Answer:
May be due to soft tissue injury

Detailed Answer:
Hi
Welcome.
I have gone through your query.
Is there discharge from nipple?
Is there any lump that can be felt?
Is there any change in the color of the skin over the area?
Since when you are facing this problem?

Based on the information provided by you it might be due to soft tissue injury to the area.
Presently for symptomatic relief:
Take mild pain killers like acetaminophen or ibuprofen.
Cold compresses to her breast will be useful pain relieving measure.

If the pain persists or increases intensity or frequency, I would suggest you to see general surgeon for examination and if on examination if they feel they may ask for mammogram.

This provisional advice provided by me is based entirely on the input provided to me. I would suggest examination by the doctor.

I hope this suggestion might be of some help to you.
Take care.

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