Pets cannot tell us when they are feeling unwell. Many serious problems begin as subtle behaviour changes, such as increased thirst, reluctance to exercise, moping about the house and many more. Cats in particular will often only show signs of chronic pain by becoming quiet and withdrawn. Most changes of behaviour occur for a reason and may indicate a health problem.
The moral is: if you are worried that your pet is 'just not right', get them checked. It may be ill and the illness will probably be treatable.