What Causes Rash On Back, Neck And Behind Ears?

Hi, I have recently changed shampoo and condt

and i am wondering if that can cause a rash on my back, neck and behind my ears. I dont have any of the little bumps on my head under my hair, but i have fairly long hair that rests on my back when its wet..? The rash isnt really colored, its just a lot of bumps of different sizes