Are you giving her free choice hay? I'd not, that is where I would start. You can also add beet pulp and stablized rice bran to her diet of free choice hay. The rice bran will give her a healthy source of calories and fats, while the beet pulp is extra fiber and can be fed even as a subsitute for you hay rations.
Also, I know you mentioned she doesn't have worms, but have you had a fecal done recently to confirm that? And what about checking her teeth? These are all things you have to look into when a horse begins to loose weight, IMO.