You have taken a good job with her external wise (shell, skin, bright eyes, etc) but the internal problems are obvious when I look at that picture.
I can see that she is over weight by a lot. You say she's 2.5 years old and 6.5 inches long shell wise? That's not very good. Females do grow larger then males, but not by that much. I don't own a female RES, only 2 males, but I figure at 2.5 she should still be less then 5 inches long if not 4.
You can also tell from her very large arms. They may look healthy to you but they are much bulgier then they should be. I figure that the front of her paw should be larger then her arm.. so in other words it should be a little skinnier where her arm comes out of her shell then gradually get bigger to where her paw is. From what I see in your picture, I simply see a very large arm that has only one shape.
But no worries, just act quickly. Your turlte will beg, you will walk away. She's 2.5 years old and a female so I don't know exaclty what to feed her now (as my 2 males are 7 years old). I figure feed her every other day. Try for more greens like someone suggested.. less protein of course. You can try carrots and other vegetables.. fruits are okay but not as good as vegetables because of stuff like they make the water acidy or something technical like that. Just feed her small portions for about 5 minutes.. feed food then wait, and give some more a bit later. Hopefully she'll poop then you can stop feeding her... I've never dealt with a overweight RES but I figure just feeding her less and less frequently will help with the problem.
Otherwise you have a nice bright eyed slider and keep her that way. 