I had the same problem and it went on for years, I would wake up with claw marks all over my boobs from scratching them so aggressively in my sleep. I finally came to the conclusion it is toxins in the breast. I bought some nipple suckers and brown discharge came out, I now try to massage and express them at least once a month and the thing that truly was a game changer.. Epson Salt baths..it pulls the toxins out.Make sure you have the hottest water you can stand and add a few cups of Epson salts and fully submerge your breasts for at least a good 20 minutes. I hope this works for you as well as it did for me. My breasts rarely itch at all anymore and if they do I know it's time to detoxify.Also if the itch is too much to handle, apply some cooling hemorrhoid cream, it stops the itch in an instant. Good luck to you.
I don't know what causes it either, but I've experienced it too. I'll be interested in the answers you get.
I don't know what the cause is, but for the past few months I have been experiencing the same thing! At first I thought that it may have been fabric softener or dry skin or something but then I began to wonder if there was a bigger problem going on. I scratch across my breasts (nipples too) that I leave red scratch lines. I scratch so hard that I don't care if my skin bleeds! Hopefully you get an answer soon. I'll check back later.