First, segregate her in a dog crate or similar cage so you can get more information.
In case she has something contagious take her to a warmer draft-free place.
Is she eating/drinking?
What feed do you use with her?
Can you describe her droppings?
Some birds at this age, because they are growing fast, have excessive need for vitamins. You can use poultry vitamins in water or baby vitamins (no added iron) so that they can replenish their tissues and so that the Vitamin D can assist calcium in depositing in the legs and pelvis.
It might be a trauma- check for cuts and bruises. Vitamins will not harm her in this case.
Now, there are several diseases related to lameness in birds, let's hope your pullet does not have any, but have there been any illness or deaths in the flock?
Any blood in droppings?
