There are a lot of things that can cause "hive-like" lesions on a dog's abdomen--Allergies to many things in your dog's environment, flea or lice bites, infections, just to name a few.
If your dog is an outdoor dog, look also for any new plants she might have gotten into or for any biting ants.
A new shampoo could cause a skin reaction, especially if not rinsed thoroughly.
Without knowing what caused the lesions, I would advise that you simply clean the abdominal area with cool water and possible a very mild soap. If they last longer than 1-2 days, you should have the dog examined by your veterinarian.
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