She has the body stance of a bird with coccidiosis. And the age is right.
I would begin treating the entire flock with Corid ASAP.
I would direct drench this pullet with 1/4 cc of straight, 9.6% Corid for 3-5 days in a row.
Start all of them on Corid water mixed at a rate of 2 tsp 9.6% Corid to one gallon of water. Put a fresh solution out daily. Make this the only source of drinking water for 7 days.
After 7 days, give them Corid water at a rate of 1 tsp per one gallon water. Again, put out a fresh solution daily and continue for 5-7 days.
Do not offer any vitamin supplements during treatment.
After treatment, you can offer a mash of their chick feed made with water and a little buttermilk. This will help heal the gut. I would offer than for about a week.