Where, specifically, is she itching when she does? Also, when you mention the back legs....is she chewing/licking her back paws (or front)?
Given this very suddenly happened and the red bumps, I do have to wonder about a pest or some kind of contact allergy (like a plant or something?). I wonder if she got bit by fire ants and is very irritated from it (?). How long ago was the trip, btw?
I would skip the oatmeal shampoo as, despite the myth, it's actually not good for dogs w/ skin issues. Instead, I'd try a medicated shampoo like this one:
Amazon.com: SynergyLabs VF Clinical Care...
You may even want to consider this spray:
Amazon.com: SynergyLabs VF Clinical Care...
One thing you might try is giving her Benadryl 3-4x a day (any less than that is not effective) and see if it works. If it does work, then you have a clue that it could be a histamine reaction (allergy reaction) to something. If it doesn't work, then I'd probably be soon taking her to the vet.