Cholinergic urticaria is a form of inducible urticaria, overlaps with physical urticaria. Mixed types are often found.
Urticaria are produced by changes in body temperature esp sweating (eg exercise, emotion) but also passive warming eg bath. Usually starts on the upper chest or neck, may initially be patch of erythema or more typically 2-5mm punctate papules with erythematous halo. If severe, can look like angioedema. Respiratory symptoms often reported but not much evidence of actual bronchospasm and rarely cardiovascular effects, cf cold urticaria. 
Often seasonal, with prominent symptoms in winter (suggesting that temperature differential is important).
Diagnosis can be made by increasing body temperature by passive warming by 0.5 to 1.5 degrees, and demonstrating rise in histamine levels (with or without urticaria). Intradermal Methacholine (100 units in saline solution) produces the typical lesions in about 1/3.
Choice of clothing is thus important, using layers to regulate warmth closely, encouraging skin exposure.
Prognosis isn't great. Half of children will still have symptoms after 4 years.