Allergy. Lots of different meds can be used. Antihistamines are very common but stopping the medicine (or other thing) that causes the reaction is the first thing to do, if known. Topical and systemic steroids are also commonly used (orally or injected).
See your physician. If one has an allergic reaction to a insect sting or bite then an appointment with their physician is appropriate. Depending upon what the stinging insect was, the age of the patient and the reaction that occured will determine what further workup and treatment in needed.