hmm could have a stroke.
and I would also give her some electrolytes.
Being overweight can cause overheating and panting. It can also lead to fatty liver disease. I would put some electrolytes in her water, and sometimes they will take them with the electrolyte water mixed into the feed. Here is an excellent article about fatty liver disease and overheating: http://www.the-chicken-chick.com/2013/12/chickens-obesity-silent-killer-how-to.html
1/2 teaspoon salt substitute* is potassium
1 teaspoon baking soda
1 teaspoon table salt
1 tablespoon sugar
1 gallon water
Can you feel of her crop to see if it is full like a water balloon. One of mine just started acting this way, and I found that she has sour crop, my first case. She vomited 2-3 ounces of terrible poop smelling liquid. If you want to worm your chickens, I would use Valbazen 1/2 ml given orally and repeated in 10 days. It gets all chicken worms. It may have to be ordered online, but Rural King and Southern States carry it. SafeGuard Liquid Goat Wormer or the SafeGuard or Panacur horse paste is also good. Dosage is 1/4 ml per pound of weight given orally 5 days in a row. TSC and most stores carry it.