12 weeks is way too early to begin layer feed. One of her problems could easily be kidney damage. Chickens have two kidneys, each with 3 segments. When consuming excess calcium, segments begin to either swell or atrophy. In either case, they are no longer functioning. As long as there are still...
Any problem could just as easily be viral, fungal, parasitic, nutritional or environmental.
Where are you located?
What are they fed and what is the calcium treatment?
At what age did you switch the birds to layer feed? (in relation to onset of lay)
That too but I didn't say so because I think people on here are tired of hearing me say it.
The only thing in the first post that indicates something could be wrong with the bird is the lethargy. Lethargy could be a dozen things. Most of which are not bacterial that could be cured with an...