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You could be having Cholinergic urticaria
due to stress
, exposure to sunlight, sudden exposure to increased temperature like after exercise or a heavy meal. You can take Antihistamines for relief. Do you often tend to get such blotches before. Are you suffering from emotional stress lately. Avoid sun exposure. Avoid sudden increase in body temperature by excess exercise, warm environment, tight clothes. Wear loose cotton clothes. Consult a Dermatologist
for proper diagnosis and management.