I think now that all the emotion that I couldn't show for my 2 loved ones (for a variety of complicated reasons) ALL that pent-up emotion came out when the cat died.
I had a smiliar pet experience that I told another JWD member about in a PM:
I didn't cry at my grandparents' funerals or when they died, but when my African Grey parrot died I totally lost it. Just like you, I was depressed for weeks and I didn't come out of my room for two days (except to get a glass of water). He died right after my grandfather passed away and right before I found my grandmother's letter.
I imagine that most people who hold in their emotions, for one reason or another, will also lose it when least expected.
My stepfather would never cry in front of anyone, and he always ranted about how much of a bastard his father was. When his father died, he acted like he didn't care in the least. But at the funeral...he practically climbed inside the coffin as he freaked out. I mean he FREAKED OUT like I've never seen anyone freak out in person. His brothers and sisters had to carry him from the funeral home and put him in the limo. He couldn't even walk on his own.