Why is my Goldfish Bent?

My golfish has been bent for about 3 weeks now....he tries to swim normally but he just twirls around the tank, and sometimes he has a rest on the bottom for about 10 seconds but he seems happy. He still eats even though it takes him a while to actually catch the food lol. Ive tested my water and its fine, my other 2 goldfish are perfectly normal...did he bump into something or does he have an infection or what!