First and foremost, sending prayers and positive thoughts for your filly to have no ill effects, and to recover very quickly from whatever was bothering her in the first place.
If you want to take action....
Once she is right again, you can contact your state's Vet Board to find out how to submit a formal complaint. Veterinarians are granted their licenses by individual states. It takes A LOT to get a license pulled, but my experience has been that just one complaint, presented correctly, will initiate an investigation and corrective action. It can serve as a wake up call to the vet to get their poop in a group, so to speak.
BEFORE you send the letter, call the vet and ask to get a copy of ALL of your animal's files, not just the filly's. If they say they won't, set your self up with another vet and have them request the records and get a copy that way. Keep the tube. Start a file. Have an exam done by another vet and ask for a copy of that record too. Document EVERYTHING, including the BS line she gave you about Quest being an ulcer treatment. :no: I would also record any further conversations with them, or stick to written communications.
And because I am a vindictive mother bear, I would send copies of the letter (and any response or resolution) to any equine practitioners associations the vet belongs to (AHA, etc; you can call them and ask what associations they are members of), and every bordering state's vet board. And for good measure, I might even find an animal/malpractice attorney and CC them as well. And I would not keep my mouth shut either; every person I know that owns horses would hear about it, just keep it factual.
I've been through a couple veterinary malpractice cases and would be happy to share what did and didn't work for me.
Quest really should be taken off the market, or at the very least be made an Rx only, although that wouldn't have helped you in this case. :no: Darn hard to find good vets anymore.