There's something in 90% of deodorants I try that gives me a bright red rash under my arms.
I switched to Tom's of Maine and the rash went away completely and has never come back. Although, it took a couple of years of using "regular" deodorant before any irritation came up. Now, I can lay off "regular" stick for months, use it for one day, and the rash is back but it never comes back when I use all-natural deodorant.
It sounds as though your rash is irritated, not infected. If this were candida (yeast), the steroid creams would have made the rash much worse, and it wouldn't have gone away, even temporarily. It's not unusual for people to get forms of eczema under the arms and in the folds of skin. The cause is usually unknown. Like many skin conditions, the best you can do is to treat the rash when it comes and stop when it goes. If the skin is smooth and just pink, the cream can be stopped. Most of the time, it just stops happening after a while. So don't worry, but I do suggest you consult a dermatologist for a more definitive diagnosis.
Best.
Dr. Rockoff