Glad to hear your chicken is doing better.
It's possible it could have been dehydration due to heat.
If you see the same symptoms tomorrow, you may want to separate, place on some paper towels or a puppy pad, so you can get some good, clear photos of the poop, just in case.
I would follow the recommendations in the post by
@Eggcessive - some poultry vitamins for a few days and extra protein may be in order.
FWIW - I'm not a sexing expert, but I think that Myrtle may actually be a B.O.Y. That comb and those wattles at 14.5wks...