Dear friend,
Sparky must be seen ASAP!! The "yellow" is jaundice. It could mean his liver is shutting down. He may not have eaten while you were gone.
Although overweight can cause diabetes and other health problems, it is very dangerous for a cat to suddenly lose weight. That is because a cats liver becomes accustomed to a certain caloric intake. If that amount is cut (for any reason) the liver will think it is starving and grab protein and fat from the cats muscles and organs. This can cause liver and organ failure, muscle wasting and lead to a weakening of the immunity and a disorder called Hepatic lipidosis:
http://www.marvistavet.com/html/body_hepatic_lipidosis.html
Essentially what happens is the body goes into a state of starvation, and a signal is sent out that the body must mobilize the fat stores to provide energy. Unfortunately, there doesn't seem to be a signal about how much fat to mobilize and fat cats have a lot... so it all gets sent to the liver to be converted from fat into glucose. And the liver gets overwhelmed and shuts down. This leads to nausea and vomiting, which means the cat won't eat, and the body tries to mobilize more fat. The cycle continues and the liver gets into more trouble.
The swollen stomach is serious as well. THIS IS A MEDICAL EMERGENCY where Sparky needs immediate care. He needs subq fluds
See if h will drink tuna water or milk in the meantime
Please take him right to a vet and let me know how he is doing.
Best wishes,
Terri
Feline Healthcare Expert
Expert in feline health and behavior. 20 years experience with cats.