You fail to state her age but I am assuming, since you posted in the ‘Older Cats’ category, that she is a senior so my response is predicated on this assumption.
If you do not see any sort of injury then this may be the onset of arthritis.
It is easier to jump up than to get down again, so at this time she has no problem going up, but coming down is more difficult so she’s taking it slower.
If you have an electric blanket or heating pad, switch it on and put your cat on it. If you are using a heating pad, be sure the pad is touching her rear legs.
While arthritis is not curable, the heat is theraputic so your cat will feel better.
Good luck, hope she’s feeling better soon.