Back pain is usually associated with either a poor fitting saddle, major trauma or pain in the limbs, usually the rear and usually the hocks specifically. I doubt she's had a trauma major enough to put her back out that badly, and as well with light work most horses are great with a saddle that fits well enough that there is no pinching, etc. If there are no white spots, no rub spots and if the dust pattern on her and the pad after use is even, the saddle is probably not the culprit.
So this leaves a hock issue and unfortunately a vet visit. The vet will likely palpate the back, palpate the legs and do a flexion test. If he finds nothing then she may have a kidney issue or something weird, but I would rule out limb pain by way if a vet visit so if that isn't what it is, the vet can diagnose it and suggest treatment for whatever is wrong.
If you are interested in doing a home test, try giving her 1g of bute orally an hour before riding and see if that helps her. That would be something also to tell the vet if it works or does not work.
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