it could be your tire pressure, one of the rear tires could be low and pulling you to the side.........
it could be your rear axle carier is not lined up, It has bolts that adjust the chain tension, these could be uneven and puling you to the side.....
it could be your tie rods up front. They may need adjusting and causing you to pull to one side......
Your frame is fine, I;m sure its not bent..... My first bet would be the low tire presure. I notice it alot when I do slow whelies and the bike turns alot to one side cause one tire has a lot less air then the other.