"Unhappy at work"

After doing the same job for 16 years and being the most productive manager in that time. Work decide to move me to a area which is out of my field because they think I can turn around a none productive area and slacking area into a better area. I feel I have been hard done by with no consultation what so ever and just told your doing it and you will be fine. I have gone to the doctors about worrying about this new role and not sleeping and since then I have been tested for Diabetes type 2 and this has been confirmed after tests. at the moment I have a sick note for a month and hate the fact that I have to go back to work into a new role to which I don't want to do. At times I feel like dirty harry picking up everyone's mess.