Hi Lee Beth, that seems to be a lot of weight to lose without trying. I think I would rather be safe than sorry and discuss your worries with your vet. I hope it is nothing, please let us know. You and Toby are in our prayers.
Hi LeeBeth...I agree with Diane, I would check with the vet. It is probably nothing more than the fact that Toby's new diet is either lower in calories or perhaps Toby doesn't really like it...and if he eats less, will obviously lose weight. But with everything he is going through, I'd be on the safe side and check it out.
I, too, will be remembering you and Toby in my prayers. Please let us know what you find out.