There are numerous causes of lameness, the most common being a limp, when a pet lands incorrectly on its feet, trips or falls from a distance that is too short. You can follow the lameness at home for a while, but the animal should not become completely three-legged or immobile. In larger accidents, it is possible that the pet will suffer a cross or fracture injury. The most common treatment for these is surgery, in some cases complete rest and a cast or other bandage can help. However, we often talk about long treatment periods lasting many weeks and even months.
There are also congenital defects that cause lameness, these include hip defects of varying degrees (more common in large breeds) and patellar luxation, the latter may cause a funny "bump" on the leg, which after a while turns into normal walking again. There is very little research on these diseases in cats.
In older cats, lameness, stiffness and constipation can be caused by osteoarthritis and spinal spondylosis, which are progressive diseases. The form of treatment is a painkiller, usually gabapentin, which the pet has to eat for the rest of its life.
Take your pet to the vet if...
the limp will not go away in a few days
the animal is fully three-legged
character changes radically
it doesn't jump like it used to
movement decreases or changes
.... it would be advisable in such cases to approach Orthopedics