What To Do If You Don't Know What Stung Your Dog But Is Swollen?

The best thing to do for insect bite hypersensitivity is to take it to your veterinarian.  They can use injectable medication--Benadryl and steroids--to immediately have an effect on the swelling. 
The most serious reaction-anaphylactic shock occurs very shortly after being stung.  This is a life threatening condition and needs immediate treatment.  Dogs will collapse, become weak, vomit, and their gum color becomes gray or white.
You can start Benadryl at home.  The dose is 1 milligram per pound of body weight give by mouth three times a day.  You can do this for the next 2-3 days.  Do not use the Cold and Sinus variety.
Poisonous snake bites should always be treated as an emergency and the dog should be taken to a veterinary clinic.

Go to your vet really fast.My dog was bit by a snake & she was so swollen around her mouth.We rushed her to the vet one night & our vet gave her a shot.This saved my dog's life.

First you have to catch your dog to be able to treat it.  What about bribing with his favorite treat?  After you've caught your dog you may want to muzzle him.  If he's in pain or frightened he could bite you.  Dogs can take Benedryl.  My 10 pound poodle can take a child's dosage.  So first thing I'd do is to administer a dose of Benedryl or similar antihistamine.  Then examine the bite to be sure it is a sting.  Do you see a stinger still embedded?  Flick out the stinger with the edge of a plastic credit card or gently remove with tweezers.  Administer Benedryl regularly and watch your dog carefully for any adverse reactions to the sting.