But doctors are now getting a better handle on it.
A new review shows it's related to disturbances in how the brain processes pain and sensations.
So the pain isn't actually coming from the area that hurts, but from the brain and spinal cord.
"They've often gone 10, 15, 20 years going to different doctors with pain in different areas of their body," says Dr. Clauw.
Many patients are given narcotics and opioid drugs to control pain - but that can backfire.
" These drugs have never been shown to be effective in fibromyalgia, and there's some evidence now that these drugs might actually be making people with conditions like fibromyalgia worse, says Dr. Daniel Clauw, of the University of Michigan.
Dr. Clauw led the review, published in the Journal of the American Medical Association.
This new review shows the best way to treat the condition is with behavioral therapy, exercise -- such as Tai Chi and yoga.
And there are now 3 FDA-approved medications specifically for fibromyalgia: Lyrica, Cymbalta, and Savella.
Doctors have also learned that the first signs can show as early as childhood, and it can affect men too.