Why is my unspayed cat losing weight, panting, and lethargic?

I would recommend getting her in sooner than February. Weight loss usually means something more than just not liking a new food, such as intestinal parasites, endocrine disorder (such as hyperthyroidism), kidney disease, etc. The greasy coat could be from the lack of grooming (either because she doesn't feel well or it is too painful to do so - oral pain or arthritis), but could also be cause by hyperthyroidism. Here is some helpful info on hyperthryoidism in cats: http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=516

Yes, indeed a cat that is not spayed and has to spend a lot of energy by going through heat will lose weight. If you do not intend to breed her, you should have her spayed, especially since cats will stay in heat until they get bred. She will be happier when she does not have to go through constant heat.

I really think you should video tape her "heavy breathing" so your vet can see it. When you take her in for an exam she is likely not doing the behavior so your vet just sees her as normal. I think the vet seeing the behavior on video will help a lot with diagnosis/recommendations. X-rays of her heart and lungs would be a good idea as any problems with either of those organs can cause breathing issues. Sometimes the problem is not evident with just a physical exam. Finding out the underlying cause for her abnormal breathing is best. Heavy breathing as you have described in not normal in cats. As for helping with weight loss I would say she should be on prescription weight loss diet like Hill's r/d or w/d; you need a prescription from your vet. That being said, if she is 10 pounds as you stated above, then that is a normal weight for most cats. I personally would not be worried about a cat being overweight until the weight is over 13-14#. A cat that is 10# would not have breathing issues due to weight. Are you sure you are not just seeing the flap of skin on the belly that happens after they are spayed? Many people see that and think their cat is overweight. I think you need to discuss this more with your vet.

Based on what you are describing I would be concerned that something is going on with Cupcake. At her age I would be concerned about 3 diseases in particular: diabetes, hyperthyroidism, and kidney disease. All of these diseases can cause lethargy, anorexia, and weight loss. I would recommend taking Cupcake to the vet to be checked out. Your vet can run some simple lab work to determine if she has any of these diseases. These illnesses can also depress the immune system and lead to additional illnesses like upper respiratory tract infections that would explain the sneezing and watery eyes. Hopefully your vet can determine what is going on so you can start treatment and get Cupcake back to herself. Best of luck.

Poor Precious! It sounds like she's quite sick. Open-mouth breathing or "panting" in cats is not normal, and usually indicates a serious breathing problem or some type of severe stress. Acting lethargic and not eating are very vague symptoms that could be caused by almost any kind of illness, so it's hard to say what might be going on without being able to examine her and run some tests. It does sound like this may be a life-threatening problem, so I would recommend taking her to the vet today to be seen. If your regular veterinarian doesn't have Saturday hours, I would take her to your nearest emergency clinic. The vet there can do blood work, x-rays, or other testing if needed to help diagnose the problem and see what kind of treatment is needed.

Nutritional issues, metabolic disease and other underlying conditions could all cause the strange eating habits and hunger, and problems with the heart or lungs (as you mentioned) can definitely cause exercise intolerance and panting. It is a good idea that you're bringing her into the vet so she can have a complete exam. Your vet will also likely recommend bloodwork and some X-rays to help narrow down the possible issues.

I am very sorry to hear about Evie! There are many different conditions which can cause these symptoms in cats. Some of these would include diabetes, kidney disease, liver disease, heart disease, infections, intestinal cancer or other cancers, and more. Without doing an examination and getting more detailed history, we cannot give a more precise answer. I would highly recommend getting Evie to the vet as soon as possible. They can examine her and will likely recommend some testing such as bloodwork to try to determine what is causing her illness and if it can be treated. Best of luck to you and Evie.