One night my dog suddenly appeared with swollen eyes (REALLY swollen) and welts on her face and back. She had been inside with me all night and the only thing out of the ordinary was a new rawhide I'd given her. I gave her some benadryl (1 mg/lb) and the swelling went down a bit overnight. The next day she developed a red rash all over her body and had incredibly itchy and red ears. I took her to the vet and the vet confirmed my suspicion: Allergic reaction. She was given a benadryl shot, prescribed 2x/day doses of a steroid (prednazone) and 2x/day antibiotics. The benadryl shot and prednazone worked almost immediately, within a few hours she was looking better.
Are any of the welts looking like cuts or wounds (as in your dog getting into a fight with another animal?). If not, you might want to very gently bathe her eye and the welts first with a cloth soaked in cool water. If you have any Benedryl lotion, it would help to put some on the welts, although the eye area might be risky (if you accidentally got it in her eye). Also -- if you have Benedryl tablets around, and if she's a medium to large size dog, you may want to wait and see if she's still looking bitten up in a while (the bites and welts haven't improved any), and try giving her 1/2 of a tablet.
If the dog is improving, and you can wait till the morning, I would try to get her into a veterinarian's office first thing -- just to be sure she's OK. I'm wondering if she might have encountered a ground nest of bees, instead of ants.
I hope all turns out well, though, and she's all right.
My dog has welts and bleeding