Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your wee one’s situation, and wanted to help.
Now I have to say that I am quite concerned about your lass. For you to see a sudden paling of the comb, weakness, collapse, and deterioration in one bird is highly suggestive of an underlying anemia (low red blood cell level). This can arise for a range of reasons (ie parasites, mites, viruses, bacterial infections, tumors, diseases of the bone marrow, etc) but for it to come on so quickly without any warning signs or other birds affected, we would have to be most concerned about internal bleeding for this bird. This could arise due to trauma, toxin exposure (ie rat bait, onion/garlic toxicity, lead, zinc, etc) or in some of our more virulent disease processes.
In this case, if she is nearly dead, then I must warn that she is likely going to be beyond treatment and humane euthanasia may be the kindest option. If she has had such severe blood loss, then she would likely require a transfusion just to survive diagnosing the cause of her severe sudden anemia. Therefore, again euthanasia is likely to be the ideal choice for in regards ***** ***** And if you do choose to do so, while it is not nice to think about, you may want to consider submitting her for post mortem. If you speak to the vet, they may be able to perform the autopsy in their practice. Alternatively, if you live near a vet school, vet lab, VLA, or agricultural college, they too likely can help you in this manner.
If the vet performs the autopsy and cannot find an obvious visible cause of her death (which would be surprising with her symptoms), then they can collect samples to submit to the lab for the pathologists to evaluate. The pathologists will be able to examine the tissues under the microscope and determine the causative agent that lead to her death. As well, if bacterial or viral causes are suspected, these can be cultured to determine what is present and what treatments options there may be for the remaining birds. This will both give you closure on her loss, but also help you know if this is something that threatens the whole flock. And once you know the causative agent, you will be able to potentially protect them effectively.
Overall, the signs you have described are suggestive of severe blood loss and anemia. For this to come on so quickly and affect only one bird, does make issues like trauma and toxins highly suspicious. And while I do appreciate she may still be fighting at this point, if her signs are this severe and progressing this rapidly, then it may be best to end her suffering and perhaps have her checked to make sure this isn't a risk for the rest of your birds.
Please take care,