Muscle pain (called myalgia) is often less severe than bone pain, but it can be very bothersome. For example, a muscle spasm or a calf cramp (a sustained and very painful muscle contraction) causes severe pain. Pain can occur when a muscle is affected by injury, loss of blood flow, infection, or a tumor. Polymyalgia rheumatica is a disorder that causes severe pain and stiffness in the neck, shoulders, upper and lower back, and hips.