It seems that Cameron wore a revealing bikini bottom in Hawaii recently and the back of her right upper thigh / buttocks was visible. Photos were taken and it looks like she has a large wide scar there.
There is a site in which the picture of the scar is of better quality:
At the blog at which the post originated, the talk was that she had had liposuction. I very much doubt this is a liposuction scar. It looks lik a traumatic scar or a scar from the removal of a skin cancer (maybe melanoma). It looks like some kind of scar revision would be in order though if it is what it appears to be.