Our 12 year old female cat has begun to flick her tail constantly back and forth and has lost weight. She is eating and drinking normally. She purrs and wants to be petted and brushed. She looks uncomfortable at times when she's sitting or laying down, and will flick her tail even while eating.
Weight loss in any cat, especially in older cats, can indicate any number of feline illnesses, including feline hyperthyroidism, diabetes, or kidney disease to name a few. Many of these conditions are very treatable if caught early, so bringing your cat to the veterinarian as soon as possible would be advised.