I Have Been Takeing Metformin HCL 500 MG Once Per

I have been takeing Metformin HCL 500 MG once per day for 3 months and I have had joint pains from this I think. Pain is in my knee and shoulder and arm i never had this before takeing these. I just was tested again and my sugar levels are normal now but have been ask to continue takeing the pills for 6 months and see were it is then. Could these pills be causeing this pain. And why should I keep taking the pill if everything came back better?