There is an article on this app in the blog section related to health anxiety, please go through it, it's really informative and will answer your question too.
This is health anxiety . I have that and was diagnosed . It took my years but mostly the fear of death due to a disease leads to this or maybe a shocked experience like seeing someone suffering from a disease .
only advice i can give with my experience is that take full care of your health and may perform routine health checkup as and when necessasy . maybe this way one may rest assured that one is fine .