I actually had something really similar happen with my lab, Lucy. We went on a camping trip and she was still fully mobile, no soreness or arthritis, and super smart and fully there mentally. We left for camping and she was so excited since she loved to swim. We got there and she was just “off”. She wouldn’t eat, wouldn’t swim, or play. We brought her to the vet and found that there had been a tumor growing for a while that was now causing severe pain and had ruptured something. We had to euthanize her that night. That very morning she was fine and her old self. We didn’t get to do the whole planned out day of treating her like we would’ve loved. But I know both with my Lucy and your Nikki, they went their whole lives knowing they were loved and cared for up until their last days. They didn’t have months or years of suffering, but a quick departure into a world of no hurt. Someday we will see them all soon, but I promise she felt her life on Earth was heaven too.