Flea shampoos have some very harsh chemicals in them. I would not be surprised if it was a reaction directly to the shampoo.
Did you know dish soap will kill fleas on contact? I prefer plain old blue regular Dawn because it is formulated in a way that it is only an eye irritant.
If she is sowing signs of flea allergies-wow watch it close, hot spots may present they look like a giant open sore can be oozing and can appear like a hole.
I thought my dog had been impaled I actually noticed the hot spot before the fleas because I use Frontline Plus it kills the fleas but not until they bite.
If it is a chemical burn or the beginning of a severe flea allergy you will need a medication (topical or oral) from the vet.
I hope it clears up and honestly have my
that it's from the shampoo as I know the difficulties in having a dog with severe flea allergies.