It could be a tumour or swim bladder infection. Have a look in your local aquatics centre for medications for swim bladder infections, but if they don't work it's unlikely it will recover.
If your fish is unable to swim to the top of the tank or swims irrationally then it is probably Swim Bladder. This is quite common in fish and easily treated.
You say that the fish is fat suddenly. Are the scales protruding? If so, this sounds like dropsy in which case there's very little you can do to save the fish.
Pop to your nearest fish centre and have a chat with the staff there. They'll probably give a better diagnosis of the symptoms.