Hi~
How is the little gal? I know you said the vet has told you twice she isnt impacted...but to be honest it certainly sounds like there is maybe something blocking. Tarry stool are indicative of iron which is indicative of blood (old) this could mean she ate something non edible and it isn't disgesting. This would lead to blockage, and if any injury may have caused a small bleed. the watery stool is the only poration that can seep out so that is what you get...much like you are describing. I would ask for an xray. see if she has swallowed something. With their licking everything they come near, it is always a risk for lizards of any type.
Also, if there is something in the gut, not passing thru, it is probably giving a sense of fullness so she doesn't get hungry.
I dont know for sure...but the symptoms all fit. xrays can be done first, if negative, then pursue the fecal...which considering the parasite with the feeders, you'd want to do even if xray is poisitive. You said she hasnt eanten any, but are they from the same pet store or wherever you got them? You may also want to notify the vendor of the problem so they check into it and and post a notification to folks who purchase from them...
Keep us posted okay?
jackie