What is the mental disorder and illness called for someone who thinks they are sick all the time?

Hypochondria

Someone said fibromyalgia is when someone thinks they are in pain and are not. It is a recognized illness and it is very painful.

"Fibromyalgia is a common health condition marked by widespread sensitivity and pain in the muscle and joints. For many people diagnosed with fibromyalgia, the pain and tenderness is constant, but the severity changes from time to time.

The symptoms of fibromyalgia were first described in the early 1800s when the condition was called "muscular rheumatism," but fibromyalgia is still largely a mystery in the medical community today.

As such, many misconceptions exist about what causes fibromyalgia, its symptoms, and how to treat it."