A form of fungal infection. I use tinactin. Probably from sweat. I wear strictly cotton socks, and shoes that can get air through. If you can go barefoot and let the air get to your feet.
Sounds like athletes foot. Did you try Lamisil cream? Keep your feet dry. Don't walk barefoot at the gym or outside your home. Change your socks frequently especially if they get damp or sweaty. Try to wear open toe shoes as often as you can. Keep the bathroom shower and bath clean.