Many types of cancer are located behind the nipple, and when they grow they may cause changes to the nipple structure. One of the most common nipple changes is inversion or retraction. This happens because the tissue behind the nipple remains deeply fixed to the tumor as it grows, and it ends up pulling the nipple to the inside.
Nipple retraction is common in some women as they go through ovulation, and you should be able to detect a cyclic pattern when that’s the case. If you have noticed recent changes on the structure of your nipples that are not associated with your menstrual cycle, you should talk to your doctor and follow his instructions to get medical advice adapted to your case.