Sick Slider !

I have a pair of red ears, one of which has recently taken to not eating very much, shedding excessively and pretty much hiding in a corner of the tank all the time. The tank is heated so it's not trying to hibernate. I rarely see it 'sunning' itself on the ledge under the light though. Their water is clean, they have supplements too and UV Lights to keep their shells firm.......I have NO idea what is wrong or what to DO.

I've had them both for several years and am absolutely sick with worry.

HELP !!!