I would ask your vet to run a senior blood profile (also called a healthcheck plus) - This will help rule out certain diseases older cats are prone to. Kidney disease, liver issues, diabetes, and thyroid. When reading your symptoms, it almost sounds thyroid related (only a veterinarian can diagnose this for sure). Usually cats with hyperthyroidism tend to lose weight quickly, eat constantly, and become weak especially in the hind area. They also look unkempt. Here is a webpage that describes thyroid issues.
http://cats.about.com/cs/healthissues/p/hyperthyroid.htm
again, this is only one possibility, but having the bloodwork done will definitely narrow down what it is.
Hope this helps! :)
Medical emergencies should been seen by a Veterinarian immediately.
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