Limping Chicken

I have a chicken, who is probably about 18 weeks old. When she was a little chick, we had trials and tribulations with spraddle leg, but in the end she came out of it ok, other than the occasional slightly gimpy day. Yesterday, she seemed to be limping more than usual, with a very stiff left leg (which is the one, if there are occasional difficulties, that bugs her now and then). Today she doesn't want to put any weight on it at all, and gets around by hopping on her right foot, or by occasionally touching just the very tips of her left toes to the ground. Her left legs looks a bit thicker than the right, and there is no roosting reflex in the bad foot, but I don't see an obvious slipped achilles or anything. No visible signs of damage, no heat radiating from the sore leg. What could this be, and what should I do??