What could cause thigh pain 6 months after total hip replacement?

Many factors: Sometimes the difference between the stiffness of the femoral stem and the stiffness of the surrounding bone can create a stress reaction at the tip of the implant which would be felt in the thigh. That being said, any pain following joint replacement should be evaluated by your surgeon.

End of stem: Some longer hip implant stems cause pressure on the bone in your thigh. Have your surgeon x-ray and check into this possibility. Also a loose implant can cause thigh pain.

Thigh pain: Agree with dr buechel, also consider implant loosening, infection, stress response, referred spine pain, periprosthetic fracture.