I am so sorry this happened. And when we lose a loved one so very quickly like this, there is no time to adjust, and it comes as a terrible shock.
There must have been a physical cause. You could maybe ask one of the vets here? But also, she was getting on in years. I know that is hard to understand, especially if she had shown no signs of illness beforehand. But at that age, something could happen suddenly like this.
Please don't blame yourself. My instinct about it is that it was her time to go. That even if you had started out to drive her the 35 miles it might well have made no difference. She might have died in a very uncomfortable way in your car instead, with you distracted by driving, rather than having you to comfort her at the last few moments. To a dog, that comfort you would have given her would mean more than being bundled into a car when she was dying.
I know how awful it is when a beloved dog dies. I send you prayers and all my best wishes.
I am so sorry it really hurts doesnt it to loose a pet, I doubt for one minute that you realised how severe the condition was, she was happy with you and it takes time to get over any loss,.Good Luck
I'm very sorry. Our pets are so precious to us. It's very difficult when they die. It's not your fault. I still think about my cat, Tommy, who died a year ago. I still cry.