Help for Border Collie

I have a 10-year-old Border Collie. I have had him since he was a baby. I also have a cat that I have had since May of this year. I also had a 13-year-old Golden Retriever that died a few months ago. Anyway, here is the situation that I desperately need help on. My Border Collie, Zackary, has never gotten into the litter box and eaten the cat feces until recently. For at least the last couple of weeks, Zack has been getting into it. And he has also had diahrea. He is an extremely well-trained dog and would never disobey a direct command, such as getting into it after being told not to. We are wondering why he has suddenly started getting into the litter box and why he is eating the feces. I have had cats in the past and he has never done this. But I have always had the other dog until a few months ago. So I am at a loss as to why he is doing this now. If he was upset by the passing of my other dog, wouldn't he have started acting up sooner? Does he not like the cat, and if so, why acting up now? Please help.

JesseK