Sometimes the skin in the crack becomes irritated, either too dry from hot showers or in your case, chapped from perspiration. Honestly, this will sound odd but diaper creams work quite well for that. After a shower gently apply a little Desitin or one of those creams on the area. It will hurt to touch it but it heals up in a couple of days. This is a great thing to have on hand too if you ever have a bad case of diarrhea. It helps protect the anus from all that burning poo. Diarrhea is never fun but a sore rear doesn't help.
OMG I have this exact same thing! I am CONSTANTLY cleaning myself. Bath in the morning...another one if I have been to the bathroom, another one at 2:00 after the first part of my job and yet still a shower before bed. But never less than three times in the tub somehow or another. I also recently became OBSESSED with cleanliness it seems and was wiping ALOT to get clean and then taking a bath. I think this might be my problem. any one else got an idea?