RI or Overweight Turtle?

I have a red eared slider who is swimming very VERY slightly to the left. You can only notice it if he's swimming at the wall (during feeding.) He is also making small wheezing noises, but I read that this may be because he is overweight. He WAS (emphasize the was) overfed, but I corrected his diet immediately. Is it RI or is it just because of the extra weight? Please help, my mom says she won't take it to the vet until I'm positive and 100% sure it's a RI.