Have her checked for a pancreatitis. We just lost our dog in April due to this. He always had a weak stomach, and had trouble digesting food. On occassion he would have loose stools, and occassional bouts of throwing up. He developed an ulcer that burst in April, and could not recover from it.
The Vet will know what blood test to do, and it is treatable when caught. Dogs with digestion issues can be treated with lactobaccilius ( sp) in a powder form, or plain yogurt. ( I make my own)
Bluemoon