If it happens for more than a few days I'd contact a doctor and see if they could prescribe you some cream.
So, the only reason I could think of that happening is that your skin has become, for some reason, sensitive to wind and such. Have you gotten sunburnt on your back? That could cause what you are experiencing (your skin begins to flake off and when wind disturbs the flakes of the skin it can cause itchyness... But I'm no doctor so I am not exactly sure)