My Face And Eye Is Swollen My Dog's Face/eye Was Swollen And Now She's Itching: What's Wrong?

My dog's face/eye was swollen and now she's itching: What's wrong? - my face and eye is swollen

Yesterday my dog's face / eye swollen. His face was swollen on one side. My aunt and I gave him some Benadryl and the swelling has decreased. Today she is itching / scratching incessantly. We thought that maybe some insects, as it was only swollen on one side. But like his whole body was burning, it could be something else? Recently had a bathroom (2 days). Do you think it may have an allergic reaction to shampoo and had no effect on after them until 2 days?