There are two major reasons why a domestic cat starts to behave strangely, (a) a health problem has developed to the point where its presence affects behavior partly because the cat is in pain and partly because of the disease itself altering behavior and (b) there has been a change in the place where he/she lives such as the introduction of a new cat or the presence of a new cat outside or perhaps a strange person in the home. A third minor reason (minor in terms of frequency) is a female cat is pregnant.
These are umbrella reasons for a cat behaving strangely. The difficulty comes in drilling down to find the exact cause of a noticeable change in a cat’s behavior.
Note: make sure the behavior is genuinely strange and not simply unexpected but normal behavior.
There is a very good page on PoC about the signs of a cat in pain (this link presents several pages, the top one is perhaps the best). These can be checked first and perhaps eliminated when trying to discover the cause of the change in behavior.
It should be relatively easy to decide if the cause of altered behavior is due to the presence of other cats or people because there will be visual clues. Often strange cat behavior due to a change in environment comes down to a cat’s insecurity. This will result in what some might term ‘strange behavior’ such as dragging food off the bowl to hide it or seeking hiding places and urinating or defecating on bedclothes, for example. Or it might cause unexpected aggression.
Change in behavior due to ill-health is harder to detect because cats instinctively hide pain and alter their behavior to mask health problems.
By health problems I include, old age. Major diseases causing altered behavior would include: Hyperthyroidism (hungry & active), hypothyroidism (mentally dull), dementia (confused behavior, howling at night), urinary tract disease (inappropriate elimination) to name three.
This page provides some pointers to look for when a cat’s behavior changes due to health issues.
Associated: signs of a healthy cat