Weaknesses in a cats hind legs can normally be pinned down to one of the following reasons:
- Kidney disease/failure
- Trauma - hit by a car or had a bad fall, fights with other animals.
- Septic blood
- Saddle thrombi
Most cats will eventually suffer from kidney disease towards the end of their life. It would be unusual for a younger cat to suffer with kidney failure unless they have a genetic disorder or the wrong sort of diet entirely. Although you can get drugs and medicines to combat and slow the effects of kidney failure, there is no cure. Often this can also be spotted by the cat not being able to control its urinating habits. People who have had several cats will tell you that this is normally accompanied by wobbly back legs and looking very skinny at the back of their ribs.
Trauma is definitely possible especially if this cat goes outside--hit by car, fight and falling badly. You should check the skin and fur for any obvious marks. Also, cuddling the cat and trying to move its hind legs to being totally outstretched should tell you if there is a pulled muscle or internal injury. If the cat hisses at you trying to even pick it up, there is a likelihood it is in a lot of pain.
Cats can also get blood clots that lodge where the aorta splits, these are called Saddle Thrombi. These can be as serious as clots in human beings,
Diabetes can cause hind limb signs. This is because diabetes leads to kidney and even liver issues. This then causes secondary issues which lead to toxin build up in the blood and then results in the cat being lethargic and having weakness in their back legs.
A low blood sugar can lead to weakness--this could be due to lack of nutrition or secondary to disease.
This cat needs to be examined by a veterinarian for further diagnosis and treatment.
Cats can also get blood clots that lodge where the aorta splits, these are called Saddle Thrombi. These can be as serious as clots in human beings, I would not jump to the conclusion that this is the problem, only look into this if you cannot see any signs of the above.
If your cats symptoms persist for longer than 24 hours, please get them to a vet as soon as you can.. It might just be a strained muscle, however, it is always better to be safe than sorry.