PAncreatitis is very hard to diagnose, and if I remember rightly, there is no definitive test in the UK, so it could still be somethign along those lines.
Tammy, I would be tempted to try somethign like steroids with your cat, eating next to nothing isn't good, and I dont really agree that is her age, 13 isn't that old in some cats. A/d is a good one to try, but it can be addictive and is high in fat, so some vets are fussy at giving it, or they will but only for short term. Will she drink cat milk? I normally use that with thin cats.