If any animal or person cannot poop, it can escalate into a medical emergency. A rabbit normally doesn't get constipated. So I would worry about a blockage from something it's eaten, a torsion (twist) of the bowel or an intussusception where the bowel slides into itself. You didn't mention what type of medication the rabbit is on.
What else did they do for her at the vet, gastric is a very dangerous situation for a rabbit and not a lot of vets special in rabbit care. She should have been giving IV fluids too. Give your rabbit the highest fiber food you can find and lots of hay. Timothy is great and high in fiber. It will help push things through. If your rabbit is still eating that a great sign. the medicine is probably a rabbit laxitive of some sort. A little belly rub never hurt either.
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