What could a red rash with no pain or itch be?

The red rash with no itch or pain on the sides of your neck is most likely a harmless condition called Poikiloderma of Civatte. It is usually caused by See also:
Does skin cancer after breast cancer treatment mean the treatment wasn't effective? excess exposure to sunlight and ultraviolet radiation from the sun. One clue that this is what's going on, is that usually the area on the front of your neck, under your chin, is spared and relatively undamaged. This happens because this area tends to be shadowed from direct sunlight.

In some cases this condition can be exacerbated by cologne or perfume worn on the neck area, followed by sunlight exposure.

Having Poikiloderma of Civatte in itself is not dangerous. But it should be a clue to you that you have had some excessive sun exposure and seeking a skin cancer screening from a dermatologist may not be a bad idea, just in case.

Additionally, there are other conditions that may look similar but indicate a more serious underlying medical issue, so do consider that visit to the dermatologist to confirm the diagnosis, since there is no way to be sure without a visual exam.