In healthy people, the immune system is the first line of defense against germs, viruses, and other invaders. But rheumatoid arthritis (RA) is an autoimmune disease. That means the immune system attacks healthy tissue in your body, in this case your joints. As a result, the joints become painful and swollen. You may also feel tired. Fibromyalgia isn't an autoimmune disease. But its symptoms are similar. It also causes pain, stiffness, and fatigue. Doctors aren't sure what causes fibromyalgia. One theory is that an imbalance in brain chemicals makes you more sensitive to pain. The same pressure that other people think is normal may feel tender or painful to you. Another link may be inflammation, which involves part of the body becoming swollen, red, hot and painful. It's the main problem in RA. Fibromyalgia isn't considered an inflammatory condition. But chronic inflammation could play a role. Your lifestyle, weight, and stress level may all raise your chances for both conditions.