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If the pain lasts for more than 48 hours, you need to see your vet. These little guys often have brittle bones, especially with age.
If it's still bothering her tomorrow, a vet visit will be needed.
Let me know if you still have questions.
There are a few things that could cause it. She may have a sprain or could even have broken a bone. She could have displaced the patella (kneecap) or she could have torn her cranial cruciate (ACL).
Only with a vet visit can you be sure. I hope these suggestions help!