This bubble is typically a response to inflammation. It could be due to an insect sting/bite, a chemical burn, an electrical burn, having been poked there with a stick or sharp object, or any number of things. I would definitely see your vet tomorrow, but in the meantime, you can give Benadryl in case it is due to an insect. The dose is 1mg per pound, and you round to the nearest dose. Benadryl comes in 25mg tablets/capsules. If your dog is 25#, you would give 25mg. If your dog is 100#, you would give 100mg. Anything in between, just round to the nearest multiple of 25. Let me know if you have any questions.
Best of luck.