There are several possible causes of the rash that you have found.
Perhaps the most likely cause is simply a contact allergy. This shows up as reddened skin, sometimes with bumps, and is occasionally itchy. It is provoked usually by the introduction of some new soap, shampoo, or similar product and switching to a different product will usually clear it up.
Alternatively, this might be seborrheic dermatitis, which is characterized by reddened skin and greasy scaly areas that appear behind the ears and on the scalp and in the hairline. It can often be cleared up with a dandruff shampoo or with a over the counter steroid cream.
Two other possibilities would be a fungal infection and psoriasis
. Fungal infections should probably be itchy, so that sounds less likely. Psoriasis should have thin silvery-white scales on top of the red skin and may also have joint pains or changes in the shapes of the nails.
If changing your personal care products and perhaps experimenting with a dandruff shampoo or over the counter steroid does not clear this up then you should definitely see your primary care doctor
, who can help you determine more clearly what kind of rash this is and what treatment is needed.