A sudden red and itchy neck can be caused by contact dermatitis, solar dermatitis, prickly heat and other factors. 1. Contact dermatitis: contact with allergy-prone substances, such as animal hair, cotton wool, etc., can make the neck allergic, which will lead to the neck suddenly red circle itchy. 2. Solar dermatitis: prolonged exposure to the sun in the environment, by ultraviolet radiation, may induce solar dermatitis, will make the neck suddenly red circle itching, may also be accompanied by burning sensation, edema and other symptoms. 3. prickly heat: body overweight, neck part of the folds more, and in a high temperature environment, when the neck a lot of sweat, may make the neck prickly heat, resulting in a sudden red neck itchy, accompanied by tingling, scratching will be out of the bright liquid. Neck suddenly red circle is very itchy, there may be other causes, it is recommended to go to the hospital in time to identify the cause, symptomatic treatment.