Post influenza myositis

My 7 year old son got H1N1 this past week. Despite being in the high risk catagory he did OK, he got Tamiflu which really knocked his fever down. We have had worse complications with other flues because if his asthma. He sterted with symptoms on Tues and fever on weds. By Saturday his fever was down to normal from 104F. On Sunday mornig he could not stand or walk because of calf pain. he could not dorsiflex his feet without yelling. It was scary. Children's hospital said it was Post Influenza myositis. I had never heard of this befor. He just started standing and walking today although he still has pain. Has anyone ever seen this before? I heard it is not very common. Thanks.