Drinking more than usual, ime, indicates a crop or digestive problem. Her crop may be emptying, but it's possible she may have a bit of a blockage somewhere.
Consider de-worming her. Safeguard can be found at stores like TSC. Dose is .23ml per pound of weight given orally for 5 days in a row...
You mention unknown age - how long have you had her? Any guesses how old?
What do you feed including treats?
Photo of the hen too please.
Any chance she ate something moldy (spilled feed, etc.).
Can you take a poop sample to bet tested by a vet?
I would encourage her to drink. Once she's...