I used peroxide to clean my daughter's ears. immediate after treatment she developed hives all over her back and around the hairline on her forehead. is it possible she is allergic to peroxide?

I doubt it. Hydrogen peroxide breaks down to water as it looses its extra oxygen molecule. She is not going to be allergic to oxygen or water. If there were other chemicals included in the mix or other exposures near the time, like shampoos/cleaning agents i would consider them the likely agent.

It definitely sounds like she had an allergic reaction. Serious allergies to hydrogen perxide are extremely rare, but there is always someone who is allergic to something. More likely the allergy is related to something else. But you should take the bottle and anything else you may have used to apply it and any detergents used to clean her with your daughter to an allergist for further evaluation.