What Causes Burning Sensation In Scalp?

My scalp burns when I'm out in the sun even for a few minutes so have to wear a Tilley hat. I know it's probably a fungal infection, because I have to wash my hair every few days otherwise get burning, itching. I use a all natural tree tea oil shampoo (which helps). I'm post menopausal and my hair is thinning. Could a food allergy cause this?