Frequently itching can be a localized response to an irritant --dermatitis.
See if your clothes are rubbing, and check your detergent you wash them in-- have you changed it. Also, fabric softener and dryer sheets. Then see if your body lotion or creams have changed. Check any other sources of contact with your nipples and any agents they may have on them - hand lotion, soap, etc. See Dermatologist