i have to say i'm not too happy with the diagnosis either, mainly the methods i guess. i mean, it may be GI but without some tests to rule other things out, how can they say for certain?
has Ashley produced anything in the litterbox in the last 24 hours? if not, i'd be concerned about blockages? maybe she ate something she shouldn't have? or constipation? etc.....
if she hasn't produced anything for the last 24 hours I'd be back at the vet first thing in the morning.
mind you, my Yoda recently did something similar.....frequent trips to the box, producing little if anything, and even making herself vomit from straining. x-rays were the first thing my vet did to make sure she wasn't blocked in any way. after ruling out the basics, yes she was diagnosed with a GI problem, given a battery of drugs and sent home on metronidazole and a nice big feeding syringe to sustain her with but i'm just saying, if it's not that and kitty is constipated or blocked for whatever reason, that can get dangerous very quickly. keep a close eye on that litterbox.