Osteoarthritis -- generally age-related wear and tear to cartilage that causes bones to rub together -- is a common source of joint pain, but it's not the only one.
Stiffness and swelling of the joints may be caused by lupus, a disease that cycles through periods of flaring up and remission. Other symptoms of lupus include fatigue, hair loss and fever.
Hepatitis, a condition that affects the liver, also claims joint pain as a symptom. Need a good reason to get that joint pain examined by a doctor? Hepatitis is responsible for more liver transplants than any other condition [source: MedlinePlus]. Many other infectious diseases -- such as measles and chicken pox -- can also cause joint pain.
Then again, it could be arthritis, but a more serious form of it: rheumatoid arthritis. Rheumatoid arthritis is an autoimmune disease, meaning that your immune system goes haywire and attacks your own tissue. This causes inflammation of not only the joints themselves, but of tissue surrounding the joints and even of other organs in your body. The result is pain and the breakdown of your joints. It's important to get medical attention as soon as symptoms present themselves to limit damage to your body -- while medications can alleviate discomfort and swelling, tissue damage is permanent.
See the next page for lots more information about mysterious pains.