I'm so sorry for your loss. As mentioned, birds evolved to hide illness from predators who would target them, and from the flock who would kick them out to keep the test of the members healthy. Its not a choice its encoded into them. So we don't know how long they have bern sick, usually they only show it when things are very very serious.
Monitoring weight, is one of only ways to try and pick up issues. Its not perfect nor will checking weights catch all health issues. It just mentioned it because of how birds hide being ill, and this is one of our only tools.
As you described, your bird was in critical condition, you did right to try and get emergency care.
You rescued this girl and helped her. With you her life was so much better. Her poor start in life might have had long term effects on her health
" Vomiting in Birds
Rated as serious
It is important for bird owners to understand the difference between vomiting and regurgitation. . Unlike regurgitation when the food comes up intact, vomited food is digested or almost-digested, and will be expelled in some liquid form. The bird will forcefully shake its head from side to side before it spits out the liquid. The force of the vomiting may send the liquid to all parts of the cage.
Causes of Vomiting in Birds
Bacterial infection – may be caused by spoiled food or old water
Parasites – worms, hexamita
Poisoning/toxins – such as plants, toys with lead or zinc
Intestinal or esophageal obstruction (blockage)
Yeast infection - Candida
Virus
Liver or kidney disease
Medication side-effects
Cancer in the GI system
Diabetes
Motion sickness
New diet
Fungus
Trauma
Allergies
Behavioral/unhappy with cage/noise/people "