I agree. Your cat really needs to be taken to a vet ASAP. I can't even begin to guess what would cause her back legs to give out. The weight loss I've got some ideas about-diabetes, hyperthyroid disease, and kidney disease. All 3 of those conditions cause weight loss. The legs giving out-I just don't know.
Please come back with an update. Your kitty will be in my prayers.
Hmm...sounds like she'll need to be taken to the vet. Diabetes is common in older cats, and it causes what you described although that's a decision the vet should make. It could be determined by performing a glucose test, which isn't expensive at all.
Other than that, did you check her leg? Any visible wounds? Is she active? Could the leg be broken?
Your best option is to take her to the vet. Please come back with updates, and I hope your kitty is well. Just remember, that if she indeed has diabetes, that this isn't a death sentence. Let us know if you have any questions and we'll gladly try to help you as much as we can.
PK.