I'd listen to what your body is telling you rather than what the surgeon said, the pain is telling you its to much, all that stress on the hardware with no fusion supporting it doesn't sound good plus theres all the stress going on the disc above too. Read countless stories of hardware breaking ,screws coming loose and disc blowing above and below the fusion i cant believe he cleared you for a pysically demanding job at this early stage with no fusion as well.
I went to see my family Doctor for advice this week even though i'm still seeing probably the same surgeon as you it sounRAB like. My family Doctor was very helpful maybe yours could advise you to what to do next, if its really hurting bad i'd go the ER.
Keep thinking i,ve read the most ridiculas thing on this subject but it keeps surprising me and turning up with worse. PT might have give you extra tips on moving heavy things too its not all excercises, i'm sure you know you lift with your legs with doing the work you do but your surgeon doesn,t know that you know.
good luck and take care