I have exactly the same symptoms but with a couple of differences. I am 64 years old and male! I cannot find a solution although it does cure itself after a week or two. Its an uncomfortable condirion, particularly the underarm area. I use baby talc to stop it chafing which helps a little. I have been unable to find any solution on the internet, most questions remaining unanswered, as was yours gemgem.  Perhaps we should start a self-help group? There might be thousands like us!  Keep smiling, Dolphin Tom.

I have found that using 'after-sun' treatment helps a lot. It removes the heat at once and the relief lasts a good while. Then re-apply, especially before bed. You have to stop using deodorant until the condition clears so you might have to wash during the day. If you do re-aaply the after-sun lotion. Good Luck. Dolphin Tom