Hi Alan,
Seen this many times, don't worry she isn't developing curl or anything as sinister as that. Its usually nothing more than a bit of constipation, does she make erratic forward or even back and forth as if to scratch her belly on the bottom? If you haven't already done it raise the temp a little, often helps to clear them out.
Any new substrate can temporarily irritate the pelvic fins (feet) and underside of a ray, remove all the sand for now till the red subsides, so long as the water parameters are checking out ok it will be fine.
Do "not" add chemicals!!