When you treat birds with antibiotics, particularly if for a digestive issue, you *must* replenish the good bacteria that are unfortunately killed with the bad. To do so, you give "probiotics" - some substance containing live beneficial bacteria which will recolonize the gut.
The common choice (unless you're medicating with mycins or cyclines) is plain yogurt. 1 teaspoon per day per bird during medication at the opposite time of the day from when the meds are given initially. Then every other day for two weeks.
If you use cyclines/mycins, then you must use a non-dairy choice. For that, it's easy enough to find acidophilus tablets/capsules from the grocery store (crushed and mixed with water into a paste), or a livestock probiotic containing live bacteria such as Probios or Fastrack. Make sure they contain live bacteria, not just by products. Live Bacteria will either be denoted by CFU and a number for colony forming units, or will literally say Live Bacteria.
Your bird likely had a secondary bacterial imbalance due to the initial illness and antibiotic treatment. She developed sour crop because the bacteria weren't there in her hind-gut area to keep the digestive tract running well. It backed up causing her crop to sour. She passed it up (as birds don't technically vomit and chickens don't regurgitate).
You'll want to make sure the crop is empty. Then she must remain on easily dissolved foods only for at least a week if not two. This means pellets or crumbles, mashed boiled egg yolk, yogurt, applesauce (great for cleaning that crop and feeding the newly colonizing bacteria you're providing in the yogurt) made into a daily treat mash of about one dixie cup in size. Feed it first thing in the morning when she's out of food and hungry. Then when she's done, give her only pellets for the rest of the day. NO grains, NO breads, NO solids that wouldn't fall completely apart in a glass of water.
That way she'll actually get nutrition.
Additionally, because her crop has been sour,, you should use organic apple cider vinegar (OACV) at a rate of one ounce per gallon of water. The reason it must be organic is that organic is made by bacteria rather than chemically. The bacteria are replaced once the product is pastuerized. You can see them at the bottom of the bottle, along with some other nutrients that a bird could use. The bacteria also help recolonize the gut (and in turn ward off bad bacteria and yeast), as well as providing enzymes which will help break down any undigested food in her gut from this. They also make vitamins and the OACV acts as a healthy electrolyte.
The bacteria that you're re-providing literally are the workers that feed your bird.
This should get her started and back on track. Please let us know if it does not.
And one more thing: please, despite well-intentioned vets, do not give antibiotics unless the problem is certainly bacterial. Many vets don't have avian experience and don't remember how thoroughly dependent on bacteria the digestive system is for birds. In this case, it worked - and that's good. The vet must have heard something that indicated bacterial infection (let's hope). But just remember in case it comes up again. So many birds develop secondary bacterial infections from antibiotics unfortunately because of mis-prescription of it, and a lack of thought on vets' parts (or ignorance on feedstore personels' parts) about the need for probiotics during and after treatment.
I hope all goes well.
