Depends. Yes it depends on the underlying cause which can for example related to circulation or nerves or muscles or tendons or bones.
Usually not. Leg pain can be due to any one of a large number of causes, most of which are minor; it would be uncommon for it to be something "bad, " but it can happen. There is not enough information provided to even begin to suggest possible diagnoses - it depends where in the leg the pain is, duration, what might have caused it, your age, medical conditions, etc. Without more info, it is best to see a doc.
Many possibilities. Your pain could be as a result of many different things. A thorough history and physical exam and possibly some laboratory and radiology exams are needed to determine the exact cause. Causes of pain may include: trauma (fracture, sprain, arthritis), infection, metabolic conditions (gout), circulatory, musculoskeletal or biomechanical abnormality and neurologic conditions (neuropathy).