I'm so sorry to hear of Maxie's passing. It's so hard to loose a dear pet. she sounds like she had a wonderful life with you, and whatever happened, must have happened quickly. I understand your being upset that you weren't with her, but I'm sure it didn't diminish her love for you. I think sometimes animals prefer to go off on their own when it's time. Maxie's at the Rainbow Bridge now, healthy and playing with all the other RB pups there. She'll be looking in on you.
I will be thinking of you & your family. Rest peacefully, sweet Maxie.