Odd behaviour...

The past couple of weeks my 3 year old girl has seemed to be a bit slow moving, like having to claw her way up the couch instead of jumping, sleeping and relaxing more. She's eating and drinking and to my knowledge using the litter regularly ( I have 2 cats with 3 boxes). She just underwent a massive battery of tests a month ago when we thought her breathing was laboured but everything came back normal. Any ideas? I weighed her and she's still almost 12 pounds.

So she's acting lethargic?  Is she eating normally, using the litter box and grooming herself? 

She's young for it, but did the vet examine her for arthritis?

Maybe she got a slight sprain or something like that?

A bit lethargic yes compared to my 5 year old boy who is quite active. Just something I observed while being home all day with them yesterday. She spent most of her time napping in the sun... And he spent most of his time trying to break outside ( he goes out once a day into the backyard on the harness with me)

Since all of the usual danger signals are absent, and she is eating, using the litterbox and grooming as usual, I would just keep a  close eye on her for now.  Sometimes they do sleep the day away.  However, if you notice any more disruptions in her normal movements, such as pulling herself up rather than jumping, I agree that it could be arthritis, although she's very young to have that.  You know your cat better than anyone.  I am one of those cat owners who would a thousand times rather catch something early on, even if that means making an unnecessary trip to the vet.  Also, any cat can have an off day or a lazy day, just as we do, and hopefully, that's all that's going on.