Nipple pain

What would cause on and off nipple pain in the left boob? My sister gets on and off nipple pain in her left boob. She said she can have it for a few days then goes away for a week or two but then comes back. She did see her doctor and there isn’t any lumps that he felt but she is to schedule an ultrasound.

Has anyone had this happen where it wasn’t anything to be worried about?