I was send to therapy by my doctor after my lower back fusion. and the therapist group make me lift more wait than the doctor told them to.

You have a theoretical cause of action against your therapist group but most likely a very hard claim to prove in practice.  Proving your exacerbation was caused by the lifting or that the additional weight lifted was a deviation in the standard of care could prove difficult. Also valuing this exacerbation next to your original underlying injury could be difficult as far a separate medical negligence case.  Under Illinois Worker's Compensation Law your ongoing medical issues wether being a continuation of your original injury or a temporary exacerbation in therapy should be covered in your work injury claim.  Anyone looking into this further for you should obtain the therapists records and the treatment notes of your physician to see how this incident was reported in your medical records.  Thereafter a determination could be made with you whether you wanted to approve the cost of an expert reviewing these documents in light of the obstacles to recovery. Generally a medical negligence case has a 2 year statute of limitations but you should act immediately to review this situation if you feel it is warranted.