I'm sure you know about hypochondria. But any thinking that rises to the obsessive level is usually an anxiety problem. Whether it's an illness or there's a trigger that is causing stress you usually don't experience depends on why it's going on, but when it goes on for years it's risen to a disorder. First order of business is to stop the Google machine. You can't really self-diagnose, as the same symptoms can be a sign of tons of different diseases or none at all, so all it does is increase your stress. And if you've repeatedly gone to docs and been told there's nothing wrong, there's probably nothing wrong. You might want to think back to when this all started and see if you can come up with something that happened around that time that might have shaken your confidence in your well-being. Therapy is something you should think about, because with anxiety, the longer it goes on the more entrenched it becomes. Learning some relaxation techniques such as meditation can help. It's not easy to stop obsessive thinking, but you can do it. With health, though, you have to be careful not to go too far, because all of us will get health problems at some points along the way if we don't die young so you do have to learn to separate what's real from what's just life's little torments. Just wondering, did this start when you had your child?
Health anxiety is real. It plagues many people. https://www.health.harvard.edu/mind-and-mood/always-worried-about-your-health-you-may-be-dealing-with-health-anxiety-disorder So, treating it as anxiety very often helps. What exactly you do may depend on how much it is impacting your life. Some may just work on improving their inner voice, exercise, sleep, etc. which improves anxiety. Others that are impacted more by health anxiety may go to deeper levels such as therapy or medication.