I don’t think her being smaller is failure-to-thrive. Correct me if I’m wrong Whisperer, but this chick has always been itsy-bitsy, right from hatch? So unless they can be failure-to-thrive while still in the egg (and they can’t as far as I know), but still hatch properly & on time, then I don’t think that’s an issue here.
Whisperer, can you take her & a buddy or two out of the brooder for a bit, and temporarily put them in a different box or tub or whatnot? Choose a couple of buddies that tend to stay on the cool side of the brooder. Include a little food and water just like it’s their new home (they won’t really live there for long though.) You want to see how they act if they don’t have a heat lamp. Will they run around happily? Or will they huddle?
I think this will help you sort out whether she is panting because of overheating, or if it’s a lung issue (pneumonia?).