It could be early onset arthritis, too.
To bring in a human example, my dad was first diagnosed with arthritis at the age of 19 (I was 50), which would be a cat of about 14 months.
Both my dad and I took/take analgesics and supplements that are safe for people.
Dad played softball into his 50's (was a catcher) and bowled into his late 60's. I trundle along day to day, refusing to seize up.
Whatever you find with kitty, there ARE things you can do to help her live a long, healthy and happy life.
In some ways, I think veterinary medicine has come along much better in my life than people medicine. =)
Probably a luxating patella. It's kind of like a football knee, the kneecap that is usually held against the bones by ligaments can pop out of the slot in the bone for a few steps before the ligaments put it back in again. If you can feel her kneecap and it's a bit loose, that also might tend to confirm it. Talk to her vet about it.
Hello and Welcome....I noticed my cat doing the same thing a couple yrs ago. An xray showed it to be arthritis. Your kitty is much younger than mine making me doubtful arthritis is the issue, but you never know.
So that is where I would start, is with an xray. If it IS arthritis, there is a natural supplement you can add to her food to help her joints.
Keep us posted and best of luck. Someone may come along with another idea, but really, we are only guessing what could be wrong.