Posted 4/10/2019 11:00 PM (GMT 0)
No idea what it is! If it were me, then I'd guess mast cells being wacky. I'm not prone to rashes (despite mast cell issues) but the last time I had one it was in my underarm because a caterpillar wandered up my side and I smushed the fluff with my arm. Rashes can be so random. In case it's a detox issue, I'd do some dry brushing in the area, take an Epsom salt bath, and massage the lymph nodes under your left arm and the breast itself.
A few ideas that might help, regardless of cause:
Cotton bras and shirts
Topless time
Tea tree oil
Coconut oil
CBD oil (topically)
Quercetin (if it is a more general histamine issue)
Homeopathic histaminum tablets (orally)
Stinging nettle tea compress
Probiotics (topically)
Good luck! That's a sensitive place to be uncomfortable. :-/