What you need to consider is that if you hurt yourself on the job, your employer is responsible for you salary if you are out of work. IF you had a note from the doctor requiring you NOT to lift and you do it anyway, your employer has a good case for not having to pay your salary if you hurt yourself again. Which is a distinct possibility if you are already hurting. So the option is yours. You can try to hide your doctor's letter and run the risk of having no income if you get hurt, or stay home until you are well.