My dog died suddenly this morning. I feel your pain. I did hear a yelping type bark as well but figured he was barking at a bike rider which was his normal. I wish I could give you answers, but I'm at a loss as well. I hope like you do that it was quick. Can't stand the thought he was in pain either. May you find happiness in your memories :(
I don't have any answers to help you, other than I'm sorry. We also have a lab/border collie, who is 15, and also has no problems other than pain in the hips. Getting up can be difficult, and so are stairs, but not that bad. I know your heartbroken because shes gone, but find comfort in knowing it was fast, and you didn't have to make that decision for her. That seldom happens. I really hope it will be that way with our girl,too.