There are many possible conditions that cause pain on someone's left side, such as a heart attack, explains the Harvard Medical School. Pain can also be caused by a slipped disk, says Healthline.
Pain in the left arm and neck can be a sign of a heart attack, states the Harvard Medical School. The pain can also spread to the middle of the chest, the back and the jaw. It is accompanied by pressure and burning in the chest, nausea, and difficulty breathing. However, pain that is definitely confined to one side of the body can be a sign of another condition.
A slipped disk can cause pain on one side of the body that extends through the arms and the legs because the disk compresses the nerves, says Healthline. This can also cause numbness and tingling on one side of the body. Generally the pain worsens at night and after long periods of standing and sitting. A slipped disk can cause the muscles to feel weak.
Fibromyalgia can cause pain on either the left or right side of the body or both, explains WebMD. Other symptoms include numbness in the limbs, headaches, urinary and bowel problems, and fatigue. Although this disease is difficult to diagnose correctly, there are treatments that can decrease the pain and other symptoms.