Help-Sick Cortez stingray

I wasn't sure where to post this so let me know if its in the wrong category.
Yesterday I went to feed my sting ray and her mouth looked red and sore. Of course she's not eating. This morning it looks even worse. Her fins are starting to turn a little pinkish around the tail area. Any idea what could be going on? I have other fish and they are all fine. My nitrates and all are good. We have a service that comes weekly for this.

Your thoughts are appreciated.

Dona