I have the same problem. It has been driving me up the walls. Like you have said in your post it is not your skin it is under your skin. But I also have a few other problems with my breast. I don't know if you have read my post, but I have a rash, intense itching, pain, bumps. I have been researching on a new type of breast cancer called Inflammatory Breast Cancer (IBC Inflammatory). If your sister has breast cancer you may want to get checked, just in case. You can go to youtube.com and type in IBC inflammatory and watch the video on this and go to ibcresearch.org and read about this type of breast cancer, because it is sometimes not seen in a mammogram or and in most cases you will not have a lump. Please go to these web sites and do some research on this because it might could be very helpful to you or anyone women you know. Breast cancer does not always show up with lumps it can show up as a rash.
What is your age? What other symptoms do you have? Have you noticed any abnormal features on breast self examination like a breast lump, nipple discharge , skin changes etc?
Apply calamine lotion to hep soothe the itching. You should wear lose clothes and take oral antihistamine medications and see if it helps with your symptoms.
If your symptoms still persist after a few days, it would be advisable to consult your doctor and get a clinical examination got done.
Let us know what your doctor advises and also post us if you have any doubts.