Chilblains are usually the result of an abnormal reaction of your body to cold. They tend to develop on skin that's exposed to cold and then warmed too quickly, such as by warming cold hands directly in front of a heater or fire. This rapid heating of cold skin can cause small blood vessels under the skin to expand more quickly than nearby larger blood vessels can handle, resulting in a "bottleneck" effect and the blood leaking into nearby tissues. Exactly why this occurs in certain people is unknown.