What have I done to my back? - hurt a bit first thing, but was OK to do some work in the shed, now its taking me five minutes to be able to stand up straight! - it must be bad, I'm even missing football!
Sounds like you may have excerbated it more when you worked in the shed. Best thing may be to rest it for a while, could be that the muscles are inflamed and need a cooling ice pack or similar on them?