At this point the concern is, what bit you because you are having an allergic reaction. Can you see the initial bite or sting mark? Wash the area with soapy water and apply aloe vera gel or you can mix baking soda with water to make a paste and apply under a damp cloth. Let it sit on the area for 20 minutes. Take an antihistamine by mouth, such as benadryl, for 24 hours. You can apply cool compresses. If the swelling continues spreading, with treatment, then you have to visit the doctor for evaluation and a prescription medication.