What Causes Pain In Thigh After Bullet Injury?

My 20 year old son was shot in the thigh and the bullet lodged near his knee but did not hit any bone. The bullet was not removed. My son was basically healed with little to no pain and was walking normally. He woke up yesterday and was having pretty significant pain where he needed a cane to walk. Is this due to cold weather or could the bullet have shifted and he needs to see a surgeon again