I've had that for years. Depends for me on what kind of soap I am using and how often a shower is taken. Sounds weird but if you don't take the shower and clean down there often enough you'll get this too.
I've also heard but have no proof that this could be a prostate issue. Not sure really how that would work but ya never know I guess.
I had to deal with this when I was younger. I even saw a doctor about it. I changed soap and laundry detergent and used a over the counter jock-itch powder. That worked for me.