That's something only a visit to a vet can tell you, sorry.
She's an old bunny, but she's not ancient. Seniorcats has a current thread about her many rabbits who havemostlylivedbetween 12 and15years.
(I don't know what a 'dwarf' cottontail is, she's from wild cottontail stock?)
It may be something simple like a touch of arthritis, and the pain is making her give up. Rabbits aren't exactly fighters when it comes to things like that. A little Medacam may be all she needs. It could be anything. But even if it's 'old age', there's something failing somewhere, and you won't knowif it's something major or minor without an exam.
For the immediate future, some Nutrical (its for cats and dogs but works forrabbits) shouldhelp with her weight loss. If she stops drinking, some Pedialyte might keep her going until you can get her to a vet.
Let us know how it goes.
sas, pipp and the warren