Teeth are a likely culprit.
Are any other horses in the barn doing it? Is she on the same feed and hay as the other horses in the barn?
Here in the states, we have something called "clover slobbers" If there's a lot of clover in the hay, or in her pasture, a fungus on the clover causes excessive salivation. In the summer, when the clover is blooming, my horses actually have damp spots in front of their stall doors from the drooling. If that's what it is, it's harmless.
However, if she's the only one in the barn who's doing it, that's not likely.