Trouble with back legs.

I have a 11 yr old, inside only cat, that is a little over weight approx. 15lbs. Something happened about one month ago that made her start to walk strangely only in the rear leg(s). It's not very pronounced but I think it's only in one leg. She does not seem to be in any bad pain, if anything she seems slightly bothered by something. Eats and goes to the bathroom okay. But when she gets up on things like the sofa she claws/climbs her way up, she normally would just jump up. And she uses mostly her front claws to grab her way up. Which makes me think she had some kind of accident and it wasn't an onset of something. There was and is no visible damage to her at all. I clip her front claws when they get hung up and haven't because it seems to be her only way to get up anywhere. Also she will sit funny, abnormally for a cat. She will sit human like, directly on her butt. From all the cats I have had I never saw one sit directly on their butt. When she sits, like when you tell a dog to sit, she'll plant her butt and then backup her front feet a few steps to get into this 'weird for a cat sitting position'. So I don't know if it's something I should go to a vet that can be fixed or if it's just age or accident related.