How old is she? Sounds like kidney problems, if she's old.
Our goddamned cat is 16 and I thought for sure she would be crossing the rainbow bridge a few months ago, but we started giving her sub-cutaneous fluids and proper kidney diet and she has sort of bounced back. Very weirdly, she had become super athletic. We have been keeping her out of the upstairs using a baby gate just to limit where she pees (kidney disease thing), but it seems all that has done is serve as a gym of sorts, and now she's able to jump really high (gate has been extended and fortified several times). 16 years old and we never had to worry about her on the kitchen table or counter, but now she just jumps right up.
But she's still clearly nearing the end of the road, and despite gaining some weight back she's still super skinny and her fur doesn't have that young cat fluffiness at all. I am guessing this strength thing is more a side effect of some aging weirdness where she has become obsessed with getting out / accessing everything in the house, maybe out of restlessness or something. I am worried that it is pain that is driving her behavior, but none of the other indicators are there. Maybe it's just dementia (it happens).