Enterotoxemia has different symptoms, not prolonged weight loss and excessive thirst. It's a sudden illness that usually starts with lack of appetite, then diarrhea, sometimes bloating, and sudden death. This is something completely different with your doe. But even if she did have enterotoxemia,
penicillin is not the treatment. If any of your rabbits had enterotoxemia, they would be very sick and die quickly, usually within 48 hours. You would know they were sick.
https://wabbitwiki.com/wiki/Enterotoxemia
I really hope this vet did injections of penicillin and
did not give it orally, or your rabbits really could get antibiotic induced enterotoxemia, which antibiotics like penicillin, can cause if
ever given orally. I don't know where you live, but if you can find a better rabbit vet, I would suggest doing so.
If your male rabbit was also drinking excessively(which if your rabbits aren't very large rabbits, 38 oz. is hugely excessive) and also experienced drastic weight loss, then they did have something going on but not enterotoxemia. I would venture a guess that it was either a kidney or liver problem. What you need is a qualified experienced rabbit vet to do a blood test and xrays, to determine what is going on with your doe.
https://rabbit.org/vet-listings/