It is NOT ok for your employer to terminate your employment because of the workers' compensation claim. Then, you would have a claim for wrongful termination against your employer. However, this may not necessarily be the case in this situation and your employer may be concerned about your physical health and his compliance with any medical reports provided by the general doctor that the workers' compensation insurance carrier is working with.
Remember that the only person that can change your work status (i.e. restrictions, etc) is the general physican that the workers' compensation carrier gets reports from. I think perhaps your employer may be looking out for you, however, he still has to make all accommodations to you in your job. It is against public policy for him to terminate you or reduce your hours, etc. without justification from the doctor.
Your employer is allowed to ask your doctor about whether he accounted for the repetitive motion in the job. Your employer is also allowed to send you to another doctor to evaluate whether you cannot return. The employer may not, on its own, determine that you cannot return, after you sent a doctor's note saying that you can. It might be best for you to go back to your doctor and ask him whether he did account for the repetitive nature of the job, and if he could make that clear in the return to work note.
Your employer can end your employment for many reasons, but your WC claim can of be the only reason. It sounds like the Employer is seeking clarification from your doctor, which IS appropriate.
We offer general concepts, but you should give ALL your facts to a licensed Attorney in your state before you RELY upon any legal advice.