Check her crop first thing in the morning before she's had anything to eat/drink. If it's not empty, then address that symptom. https://www.backyardchickens.com/articles/how-to-treat-sour-crop-and-impacted-crop-and-how-to-know-which-one-youre-dealing-with.73607/
Often there's something else...
Hopefully the vet can see her soon.
Hard to know what's going on with her. She's 6yrs old, so may be starting to have some reproductive problems. You mentioned she was drinking a lot of water, but has recently stopped. Is her crop emptying overnight? If it's not, then address that...