I was just diagnosed with myositis. Does this mean I have an auto-immune disease? How serious is this?

Serious but Rxable. Myositis is inflammation of muscles caused by autoimmune attack of the muscle tissue. It can sometimes be painful but the primary symptom is progressive weakness (over weeks and months). Affected muscles are usually proximal (closer to the body) such as shoulders and hips while wrists/hands and feet are usually spared. Should respond and improve with therapy. See a rheumatologist for help.
Myositis. is an inflammation of muscle resulting from local cause. Clinically, myositis presents as a more-or-less continuous painful muscle that may be accompanied by swelling, redness, increased temperature and diffuse tenderness over the entire muscle. Therapy for myositis includes use of analgesics, anti-inflammatories and physical therapy.