Water therapy also does wonders for their strength. If she can swim (possibly alone?) it could be a big help for her also.
My mind went more to breed than actually trying to help with her possible sickness, I’m so sorry. The vitamins are a good idea also. It’s odd for her to be experiencing these issues at her age. How long has she been acting this way? Could she have ingested something that made her sick? Any possible poisons in the yard? Chemical or poisonous plants she may have eaten a small bit of?