If a woman, do not know if you are perimenopausal, post menopausal or if you are in the child bearing years. Don't know if you have had a child or if you have breast fed an infant. Do not know if this is a new change to the nipple or if it has always been this way. Will assume that there is no discharge or fever
since it was not mentioned. One nipple could be harder or more tender if there was differential friction on the nipples. Clearly, you indicate that there is a difference between one nipple and the other. I do not know the cause. There could be an infection, a blocked milk duct, a non- malignant tumor or a malignant tumor or this might be related to hormones. If the nipple is hard to the touch, i would recommend that you be evaluated, to ensure that there is not a mass causing it to be hard.