Hi, There could be different reasons such as mood disorders or obsessional doubt or psychotic phenomena or paranoid personalities.  Psychiatrist are trained to differentiate and advise treatment accordingly.

There are several possible reasons. It could be a normal thought triggered by a stressful event which would eventually settle. Or it could be an obsession in which you might have a doubt which you are aware is not true but you are unable to get rid of. The third explanation is possibility of a delusion or a firm belief in some external negative influence or Black magic happening to you. There are few other possibilities as well. The answer can be figured out by detailed assessment.