Dental/tooth infection
Insect bite
Allergic reaction to food, meds, plants, etc
Any allergic reaction can cause this....This time of year, it's normally diagnosed as a bee sting, spider bite, or snake bite.....However, if the swelling is excessive, it can effect the throat & respiratory tract and cause suffocation. Snake bites can be fatal......
Give Benadryl (Dose is 1mg. per lb. of body weight) and get to your Vet.......Karla