There's no way for any of us to diagnose what this might be via the internet. Have you asked your Vet their thoughts? Maybe you could send that picture and see what they have to say about it, if they haven't seen it in person.
I'd be very interested in what they have to say about it.
Since this post is nearly three years old I'm not sure if you will receive a reply. However, please feel free to start a new thread about your cat's health issue, pictures are always helpful.