I can confirm sudden weight loss in a cat is definitely a cause for concern. When you notice that your cat is losing weight quickly, it can point to serious health problems. Diabetes, kidney disease, and hyperthyroidism are some big ones. Feline Infectious Peritonitis (FIP) is another serious condition that could cause your cat to lose weight. It's best to see a vet as soon as you notice these changes to make sure it isn't something serious.

However, there can be more normal reasons, too. Cats may lose weight if they are stressed, maybe due to a new pet or a change in their environment. 

They might just be eating less for a while due to something as simple as a change in their routine or diet. Sometimes, older cats lose weight naturally as they age. 

Still, it's better to have a vet check it out to ensure everything is fine. If your cat is sick, the sooner treatment starts, the better. This is especially true for diseases like FIP, which can be fatal without the proper treatment.