I’m so sorry Judith 🙁 sometimes there are no signs or explanations.. It’s just there time to go.. Take comfort in the fact she didn’t suffer a long illness and she had you with her as she passed.. The vet should deffinately do some tests to see if they can understand what happened ..
I can understand your pain with the worrying about her companion.. I lost my soulkitty Buttons only weeks ago ( he’s my avatar and Nutini his shadow is lying on top of him) the pain I felt was emmense but I can’t imagine how Nutini felt .. They were joined at the hip 24/7 for nearly 3 years ever since Nutini was 4 weeks old they were never separated for more that a few minutes even their vets visits were done together. But they do come around .. Nutini suffers from PTSD so almost immediately we had to adopt another kitten.. She’s still adjusting to the new addition Mr Noodle but it helped..
We all here believe all our furry babies go to a place called the meadow.. And they wait for us there.. I explained the death of one of my cats a few years ago to my nephew.. I said “you can feel the wind but you can’t see it ” that’s what happens when our furry babies pass we know they are there and can feel their presence sometimes we just can see them..