Hi Jess, Welcome to the forum.

I would keep a close eye on your frog just to be sure. Is he eating? If so then its probably OK. It may have gotten a fright and gone into a sort of shock state. I have heard of this happening. I have a neighbor who has a young Southern bell that used to go into a seizure type state for a few seconds when its desturbed out of hiding. Ive seen it happen and it really worried me, Its legs were stretched out like you said your frog did but it was also twitching. It had done this about 5 times in the first 2 months as a frog,but that was quite a few months ago and its doing fine now without seizures. Growing and eating well too.

How many Whistlers do you have? I also have Whistlers and I love them to bits.