Ok...there are a few things that are signaling me as to why this could be happening. Purina dog chow is filled with corn, which is basically just a filler. You might consider switching brands of food to something like Diamond Chicken and Rice or Diamond Lamb and Rice. You can find a retailer near you here:
http://www.diamondpet.com/dealer_locator/ (its very economical and good for them too)
The other thing is, try splitting her meals up into 2 smaller meals. When dogs go 24 hours between meals, it may weigh heavy on their stomachs, causing instant upset.
You can try moistening the food with water before feeding also, this makes it easier to digest :)
If none of these things work, then I would think something like acid reflux...same as humans get. Discuss the problem with your vet and talk to him about using Pepcid A/C for your girl.
Hope this gives you some ideas...and hope the problem gets resolved soon :)
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