But she was palpated as well Foxhunter.
Good the vet noticed the fluid. Even when you think you have good quality hay it may not be enough for a given horse. Or their system is holding on to it to extract every possible nutrient which to me says they still need over and above in some way. Not calories but nutrient density. If her feed has changed drastically that could also account for the milk. Feeds high in phytoestrogens can wreak havoc on a system. And then there is the chance she lost one if she was carrying and her body did not recognize the loss.
Hope you get it all figured out. Sounds like she made a soft landing and will be well taken care of no matter the outcome.
Some horse people change their horse, they change their tack and discipline, they change their instructor; they never change themselves.