Could you explain what is myositis and what causes it?

Its Inflammation of: The muscles, caused by an injury, an infection, any medicine/chemical injections, bleeding ( from an injury) in or around a muscle. Best to see your pcp to be evaluated/treated. Good luck ; best wishes for 2014.

Weakness>Pain: 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. Sometimes accompanied by Raynaud's and purple/red rash around eyes and on chest