If she has a severe UTI infection, it's a possibility that's the cause of her inability to stand. If the clearing up of the brown urine and possible UTI doesn't improve her limb function and get her back to normal, there are some possible more common causes for ataxia or loss of limb function in rabbits.
https://wabbitwiki.com/wiki/Urinary_tract_infection
Most common causes are an inner ear infection and the parasite Encephalitozoon_cuniculi. Hind limb paralysis and head tilt would be the common terms to look under for ataxia or loss of limb function, even though your rabbit doesn't specifically have head tilt. It has to do with ataxia and a loss of balance.
https://web.as.miami.edu/hare/paresis.html
https://web.as.miami.edu/hare/tilt.html
https://www.dvm360.com/view/diagnosing-and-treating-common-neurologic-diseases-rabbits
http://www.disabledrabbits.com/paralysis.html
Though your rabbit is on antibiotics for the UTI, if an inner ear infection is the cause of the loss of balance or ataxia, a much longer and more aggressive antibiotic treatment (with rabbit safe antibiotics) is usually necessary. 4-6 weeks, or sometimes longer, is common, with specific antibiotics being more effective than others. I've had the most success with azithromycin, but some rabbits react negatively to it.
Medirabbit: ear infection clinical signs and treatment
When it can't be pinpointed to an ear infection, and e. cuniculi is also a possibility, rabbit vets will often treat for both until either can be determined as the definitive cause. The common meds prescribed are Panacur(for the EC), a rabbit safe antibiotic, and meloxicam or sometimes a non immunosuppressive dose of corticosteroids, to help reduce the inflammation. But steroids have to be used with caution in rabbits, due to the effect on their immune system. Reducing inflammation is important, as continued inflammation can lead to permanent cell damage.
https://wabbitwiki.com/wiki/Encephalitozoon_cuniculi
Another possibility if it's not a loss of balance but instead diminished motor function, is floppy rabbit syndrome. This is believed to possibly be due to a nutrient deficiency, but it's cause is unknown. Usually with this, though the rabbit can't move their limbs, they remain alert and responsive, and with supportive care and feeding are often back to normal in a week or two, with full motor function restored.
https://www.caldervets.co.uk/pet-help-advice/case-studies/32-marley-floppy-rabbit-syndrome
http://www.medirabbit.com/EN/Neurology/Flop_rabbit/Floppy_rabbits.pdf
Some other possible causes for ataxia or for loss of limb function, are stroke, head trauma, spinal trauma, spinal deterioration, spondylosis, cancer, other systemic infection, or ingestion of a toxin or toxic plant.