Your joints are swollen and stiff. Your muscles also ache, and you're exhausted. Are these symptoms related? They could be signs that you have rheumatoid arthritis (RA) and fibromyalgia, two health problems that sometimes happen together. If you have one of them, you are more likely to have the other. Only about 2% of adult Americans have fibromyalgia. But it's more common among people who have RA. Research shows that between 10% and 20% of them also have fibromyalgia. Experts are still studying the reason for the relationship. But they think that several factors play a role.