For the benefit of others...
I once got an itchy red soreness under the armpits which appeared after a few swims in a hotel pool. But I was also using a new deodorant which was beginning to cause some irritation, so I don't know whether it was an allergic reaction to the deodorant, or some fungi infection from the water, or both. The irritation hadn't appeared anywhere else on my body, so it made me think the deodorant had broken down the skin's natural defences and that is where the fungi from the water took hold. It did take a while and 4 visits to my UK doctor before we diagnosed the problem and the right cream to 'cure' it. So now I always travel with an anitfungal+anti-inflamatory cream - available in Thailand as 'Myda-B'. It contains Clotrimazole and an equivalent to Betamethasone.
You can buy it in the drug store on the ground floor of the Silom Centre - that's the shopping complex adjacent to Sala Daeng Skytrain station in Silom Road Bangkok. 72 Baht for a 15g tube. You may not find it in all Thai drug stores especially those prim and proper about prescriptions, but doubtless it will be found with a little determination. It comes in a blue and white cardboard outer.
In the UK the same ingredients are sold under the brand name 'Lotriderm' manufactured by Schering-Plough Labo NV Belgium and distributed in the UK by Dominion Pharma Ltd. It is only available on prescription. It carries a retail price around ┬г18.00.
The leaflet in the Lotriderm packaging describes Betamethasone as a topical corticosteroid used on the surface of the skin to reduce the redness and itchiness caused by certain skin problems, and Clotrimazole is described as a topical antifungal medicine used to tread some fungal infections of the skin.
It's worth making a note of this cream... its Magic Stuff! The cream provided instant relief and cleared it all up in a couple of days. One day you might catch some skin irritant like this yourself so if you can remember or refer to the name of this stuff you'll save yourself a lot of unpleasantness.