Consider coccidiosis as ONE possibility. Are they recent additions to your place by chance if so how long ago.. I mean weather in your location's gotta be dry.. but droppings can get in water and that's the fastest way it can spread, noting coccidia themselves are in every dropping but only a problem when too many have been ingested, and sometimes during times of stress like moves and such.
The attack after being separated is 100% par for the course, unfortunately. Next time you need to separate one, try to allow for in sight separation so they remain a part of the flock.. which has established a new pecking order without her by now. Chicken flock pecking orders are very dynamic. I just noticed your user name, Order totally matters and welcome to BYC!
First question regarding the Lameness you notice.. what are you feeding including treats and supplements on a regular basis? I see you said "added some oil into her seeds". If it isn't a formulated chicken ration then your lameness *may* well be cause by malnutrition.. ie vitamin or mineral/amino acid deficiency.. ONE possibility anyways.
Follow up.. IF malnutrition is ruled out.. How bad is is it.. limping, able to stand up? Any heat or swelling that indicates it could be an injury? Possible stiffness from being confined for treatment? Pain response when touched other than usual annoyed behavior
Third and fourth possibilities.. getting ready to lay an egg and the sciatic nerve is being pressed as the egg passes.. not super common but also not unheard of.
Marek's disease virus is one of the more common causes of lameness.. California is NO stranger to chicken disease and Southern California in 2018/19 faced quarantine and subsequent mandatory euthanasia of thousands of back yard birds in several So Cal counties due to virulent New Castle's disease. Government officials (USDA, FDA, CDFA, ??) also placed a shipping embargo of ALL live poultry to and fro that entire area THROUGH USPS that lifted June of 2019 but has been swept under the rug (or rather became unimportant and business resumed as usual like it always does) due to fear mongering of Sars-Cov2 also known as the novel Corona virus...
In other words, since I know it didn't come out this way despite what I'm trying to express is..
FEAR NOT. Hang in there, do what you can. If her journey in this life does end sooner than later.. you will know that you're better for the time you had together. Sorry you face this.
I hope you find answers and she recovers quickly!