Could she have gotten into any houseplants? (many are toxic to kitties and kittens are notorious for chewing them)...what about flowers in a vase? (same with them)
If you saw her hind legs give out and you're seeing her limping and things just don't seem 'right' to you, I'd definitely see if you can have her seen a Vet today, even if it's an Emergency Clinic Vet. I would definitely do this if I noticed symptoms that you're noticing.
Maybe she is dizzy? (something neurological)...maybe something bonked her on the head? (fell on her)
I'd really encourage you to get her to the Vet for a good assessment, better sooner than later...........I wouldn't 'wait and watch'........most often things are much easier to diagnose and treat if detected as soon as possible.
Please keep us posted.
And when you take her to the |Vet, if you don't already, please ensure you take her in the car in a proper carrier, so she's secure..........and cover it up with a towel or sheet.........cats often get very 'motion sick' from being in a vehicle as they see things wizzing by them out the window.......better if carrier covered...........and if she's got any kind of a neurological issue, this is really important.