I second the allergy diagnoses. What are you feeding her? Look at the ingredients on the bag of food. If the first 2 or 3 are not a recognizable meat source, the food is not nutritionally sound. Sure - it will fill her up, but she won't get any good from it. It certainly isn't your fault! The dog food manufacturers are clever marketers.
Most dog food sold in the grocery store and Walmart or other discount stores are full of corn and by products. Dogs eat meat.
Just say "no" to corn. we are feeding dogs, not cows or chickens!
If you would like some links about dog nutrition and foods let me know and I'll PM them to you.
Thank you for rescuing her.