A well-balanced meal plan for cats includes protein, essential fatty acids, vitamins, minerals, and other nutrients. When your kitty eats too little or too much of certain nutrients, health complications can arise.
Tuna on its own is not nutritionally balanced. For example, tuna has too much unsaturated fat and is not supplemented with Vitamin E or other antioxidants. Therefore, tuna should not be fed as your cat's primary source of nutrition.
If tuna is part of your cat's meal plan and you notice them acting out of sorts, consider taking them into your veterinarian for a wellness visit — it's always a good idea to get a baseline reading to ensure nothing serious is going on.