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That is a well spread out rash. However if the symptoms do not increase, and there is no onset of breathlessness, or wheezing, or any angioedema, you can take Tab. Claritin ( Loratidine ) 10 mg once before bed time, but do get a follow up tomorrow morning. Without any history of allergy, or any kind of exposure, getting such an allergy is cause of concern.
ER doctor can get a few tests done and check for eosinophilia, a common manifestation of such urticaria.
Feel free to discuss further,