Sick Leave

I am on sick leave at the moment.  I have a job which is a desk job and requires a lot of thinking.  The problem is I can not sit still, settle and get on with my work at the moment.  I have a lot of built up physical energy that keeps telling me to get up and do something.  The frustration comes when there is nothing physical to do.  I was climbing the walls at home in my one bedroom flat so I am staying with my Mum for a few days.  There is nothing physically wrong with me but I am incapable of sitting down and concentrating.  I am sick but I do worry that others do not understand the nature of my illness :o(.