You are suffering from a condition called Dermographism.
'Patients with simple dermographism are asymptomatic and require no therapy. Recognition of the problem, avoidance of precipitating physical stimuli, reduction of stress and anxiety are important factors in medical care. Also, scratching because of dry skin can be reduced with good skin care and emollients. H1 antihistamines are the drugs of choice.'
You could read about the condition at the following links -
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