Flea-free dog still itchy with skin bumps & dry patches. What's wrong?

Skin problems in dogs are usually secondary to allergies. Due to the her having dry skin on her backside, and the history of fleas, I think that Samantha has a flea allergy. Flea allergies tend to make dogs lose hair and become very itchy on the back 1/3 of their body. The key to getting rid of fleas is to make sure that you treat the environment where Samantha is as well as her. The fleas that you can see are only the tip of the iceberg. Most of the flea life cycle is in the carpet , bedding inside and grass outside. There is a stage of the life cycle that can not be killed with pesticides. So- it is important to treat the dog and environment and then repeat it in 3-4 weeks. If your dog has a flea allergy, it is important to keep them on a monthly flea prevention because even one flea bite will make them itch.
If your fleas are truly gone and you have Samantha on a regular flea prevention, she could have another allergy like an inhalant allergy. I would recommend a visit to the veterinarian to figure out what is going on with her abdomen. Bumps could be a sign of a bacterial infection. Your vet may be able prescribe a treatment to get her feeling better.
The thing to remember about allergies and skin problems in dogs is that there is not one magic treatment that works on all dogs. It will require team work between you and your vet to determine what works best for Samantha. Don't get frustrated if the first thing doesn't work, go back in for a recheck and try the next treatment.