If this is a new thing it could mean that she has a loose tooth or some kind of gum infection. Is it possible to get her to the vet for a dental check-up?
This is not uncommon and many cats drool when they are asleep because they're relaxed. However, as the above members mentioned check her mouth and if you're concerned it might be something else by all means contact your vet.
Boone will drool sometimes when he's content and relaxed. Usually when he hops on my lap and knows he's about to start suckling on my shirt and/or get lots of pets.