Dermatographia [dermatographic uticaria].

About 4-5% of the population have what’s commonly known as “skin writing”. The root causes aren’t fully known (though it’s suspected to have multiple causes, including nutritional problems and stress reactions), but it occurs when the skin has an allergic-type reaction to any form of scratching or rubbing of the skin, even though it’s not reacting to any antigens. The affected area is generally itchy or somewhat burning, but not very painful. The condition also does not cause a generalized allergic reaction, except for in about 50% of first outbreaks (reasons for that are unknown). 

Swelling and redness generally disappear within 15-30 minutes, but sometimes last for hours.

A Practical Treatise on Diseases of the Skin. James Nevins Hyde, 1909.