Myositis is the general term used to describe inflammation of the muscles. Dermatomyositis and polymyositis are all considered inflammatory myopathies. Inflammatory myopathies are thought to be autoimmune diseases. All of these diseases can cause muscle weakness, but each type is different. Some early signs of myositis include trouble rising from a chair, tired feeling after standing or walking and difficulty swallowing or breathing. Myositis may occur on its own or in conjunction with other connective tissue diseases.