Many possibilities. Muscle tenderness can be caused by many conditions, from a strained muscle, pinched nerve, arthritis to name a few. It's best to begin with a thorough history and examination with your physician to evaluate the cause and discuss treatment options.
Sorry. Sorry, not enough information here. There are many potential causes for this tenderness. You need to see your doctor who should be able to figure it out and if not send you to a specialist to help. Good luck.