A sad, sad night

I took this picture of Aspen today, on the way home from Pleasanton (incidentally, Max was sitting in my lap at the time). When we got back to town, I took her to the vet for some routine vaccine updates. We got home, and since she had to wait a couple of days before being groomed, we put her in the back yard. James took her some water, which she drank. Pretty soon, she laid down. It had been a pretty exciting day, so we figured she was tired. I went to the store, came home and brought her some food and some more water (with ice- it's hot out nowadays!), but she didn't seem interested. A few more times during the evening, I went out to check on her, and once her collar seemed a bit tight, so i loosened it. She still seemed ok... tired and hot, but ok. JR & Kat came over to fix our fence, & I went back out to check on her, and she had passed away. We just don't understand... she was happy & energetic just a few hours ago. I'm so sad for James... he's had her for all of her 8 years, and he just lost Denver (a 13 year old dalmation) a few weeks before we got married. We are sad tonight.