Wow your poor girl sure has a sensitive tummy! I can't say for sure, but it almost sounds like indigestion. Kind of like heart burn for humans. I'd be really cautious on introducing any new foods into her diet. Does the fish treats specifically say what type of fish is in them, or does it just say fish?
There is products on the market to help with firming up their poops, look for a product called pet flora (or flora pet, I can't remember :) )
There is no chance of a blockage of some sort is there?