There is a rabbit “disease” caused by a parasite that’s called e. cuniculi. It can affect both the gut and balance and the hind legs. (It can also cause other symptoms such as head tilt and kidney problems). This parasite is often transferred to the rabbit when it is in in its mother’s womb, and it’s belived that 50-80% of domestic rabbits are carriers. Most buns don’t get sick from it, but some do.
There are tests available for this parasite, but they are notoriously unreliable. For that reason, many vets choose to treat upon suspicion only. Treatment consists of a dewormer (Panacur) and meds to alleviate the symptoms, depending on which set of symptoms the bun presents with.
You could perhaps call your vet and tell them about this. It’s of course best if you can find a rabbit specialist – they would be aware of this common ailment of rabbits.
Here is some info about this parasite, from the British Rabbit Wellfare Association. It does not mention stomach problems, but it is now known it can affect the gut and we have a member here (Asriel&Bombur’s Bombur) who is a sufferer. http://rabbitwelfare.co.uk/rabbit-health/e-cuniculi/
Another possibility would be muscle weakness from electrolyte loss due to the diarrhea. But e cuniculi would probably be my first suspicion.