Seems like an allergic reaction to the product you shampoo with. My partner has very sensitive skin and any soap or shampoo that has perfume or alcohol in it will give her a rash. Have a look on the ingredients to see if it contains perfume or alcohol. Hope this helps

Me too, I am randomly allergic to elastic, coffee, contact lenses.

I have had this before. I basically went allergic to everything/soap/products for a while.

Is it a red lacy type rash like this?