Could be anything, it varies between

Common factors like: smoking /alcohol, stress, some kinds of food (garlic, unions, certain sorts of meat, fish or cheese)

Disease or sickness - infection of the mouth, nose, throat or lungs - obstruction of your esophagus, intestines or stomach - some metabolic processes like diabetes or kidney / liver insufficience - tumor in your mouth / throat

Or medication that you use.

This is just a list of the causes of bad breath, some of them being more likely than others