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Hello. Thanks for the question.
In a young cat like this, I would guess you're dealing with a viral infection- Calicivirus. This is a common viral infection that can get into the joints and cause a viral arthritis. It's nothing permanent, but for a week or two they will have a stiff gait and be reluctant to walk around. I typically put them on an anti-inflammatory medication, Metacam, to help resolve the signs faster.
I hope this helps, let me know if you have any other questions.