She was just under 6 years old (average GSD is 8-12).
She had, at least that we know of, one very large and one small mass in the end of her GI tract. Apparently extremely difficult to resolve, and even if a successful surgery were to happen the lifespan after is still bleak. Essentially she wasn't able to defecate.
Last Tuesday we took her to a new vet, who found it. We had taken her to our previous vet twice in about a 4-5 month timespan for issues that we now know were definitely related. We had less than a week with her after we found out.
Quick moral is.... If all your current vet does is provide pain meds and antibiotics because they don't know what's wrong - run - find another vet.