H.L.
This has actually happened to me intermittently over the years and yes, on usually one breast (not always the same one). It drives me bananas! How long as this been going on? Thankfully, it seems to only last a few days at a time for me. I usually try something topical first, like a hydrocortisone cream. After I scratch a bit, I sometimes get welts, like hives.
If it is persistent, I would definitely try to see the dermatologist. I have not gone to see one yet, but mainly b/c it disappears before I could even think about getting into to see the derm.
Good luck.