Hello scaredyandgaga,
I have been a foster home for a local cat rescue for about 2 years now. Our home specializes in kittens with distemper, kittens with severe upper respiratory and mother cat with kittens. When everyone is finally healthy they start ripping around here like their tails are on fire. We've seen our fair share of injuries. Our director has always said watch for 12 hours very closely. Look for swelling at the site, drooling, increased temperature and rapid breathing. Any of these things are are trip to the vet. Expect them to lay around and not be themselves. I've done ice in a zip-lock back wrapped in a dish towel and layed on the site for no more than 5 minutes at a time. If its a sprain it will slowly improve.
You mentioned she won't let you check it. We have used a towel to secure our fosters, just so we can get a look say at the pads of her paw. Could she have something in it?
Kittens have a way of talking the older cat into engaging in some good ol' fashioned fun! Mostly it's good for them (a little exercise never hurt anyone), but sometimes they go a little beyond their limit. Separating them won't hurt anything and your 3 year old may appreciate the time to heal. So, check her through out the night for the things mentioned above. If she is the same or worse in anyway, try to get her into your regular vet. Waiting through out the day will only cost you more in the long run if you have to take her to an animal emergency clinic. Plus, the option to do a pymt plan is NOT available there. Your kitties vet will be your best bet for that. Mine has taken half up front and agreed to a post dated check or two to be deposited at an agreed upon time. It is a tough time for us all financially, but I bet your vet will work with you if it comes down to it.
Best to you and please update as to the status of your girl!