I am not a doctor but I looked at the pictures and the rash looks very similar to the rash I get when I wear a bathing suit. Every place that it touches on my body I get that rash. I have no idea why. First I thought it may be the chemicals in swimming pool water but the same thing happens when I swim at sea so I am thinking it is something in the fabric.
By the way this started when I was 27 years old so I guess it may be something your body develops late in the game.
And I also have eczema and I get a similar rash when I use certain penicillins though I have no idea if there is any connection or not.
What your talking about could be cause by yeast. Wet bathing suits and antibiotics make yeast really happy.