Get a vet out to check her teeth.
sweet feed can color teeth yellow/brown.
Horses aren't supposed to have pearly white teeth like humans. They are constant grazers, so their teeth can look dark green or brown tinted from their food.
You should look for hooks, or alignment issues of the upper and lower molars and premolars.
Your vet can come and assess your horse's mouth.
Then you should prob call the vet..