I'd be looking at teeth as well. When she eats other things, does she move her head around funny like she's trying to manipulate the feed in her mouth? Watch that especially with any dry feeds (pellets, grains, etc.)
And to Barry -- not all horses need their teeth done every 6 months. My one girl has been here 3 years and still hasn't had them done -- and the vet HAS looked. My other girl has been here a year and still doesn't need it. I've had and seen lots of horses that go for years without floating. It depends on the horse, their age, their health and their diet. Generally, the more free range grazing they have, the less they need their teeth done.