Sick caterpillar? HELP, URGENT?

So nice of you to try and help. I've always done the same thing. The best advice I can give you is to gently put it back next to where you found it, and out of sight of birds, laying a few leaves of the tree under and over it and perhaps a small cap full of fresh water. Very few of the bugs I took in made it, they don't seem to do well in captivity even though we want to help them so much. Some of their curling up and writhing might be a reaction to being taken out of their natural environment. Whatever you choose to do, it sounds like you are a fabulous person for trying to take care of our earth's creatures. I wish you the best!