You are probably suffering from what Dermatologist call urticaria. Common causes are food allergy infection or drugs. It presents with itching resulting in formation of red rashes which we  I call hives or wheals. Although in most cases cause remains unknown in spite of a thorough search. It can be symptomatically treated so that you do not get any new hives and the treatment should be prolonged until it resolves which usually takes 1 to 6 months. Please consult me in private for further details. Allegra is very short acting. Better antihistamins are awailable.