Thanks guys. She was almost 13 years old, so she was an old kitty. I found her sleeping under my bed this morning, took a shower, and she was gone when I came back. I imagine it was just old age, which is a little comforting. I'd much rather have her pass away the way she did than, say, if she got hit by a car or something.
It's weird though, whenever I grab a gallon of milk out of the fridge, she no longer howls and begs for me to give her some. It's the little things that you miss the most when they're gone, you know?