If the tumor was effecting kidney function a blood test might have given a hint that something was wrong, but it's unlikely that she would have shown any external signs of illness. You did good by her, these things just happen.

I lost a het pied adult male last year to cancer. He was outwardly perfectly healthy one day and dead the next, the necropsy showed that he had severe tumors in every single organ of his body, but he had eaten 3 days prior to his death.

All you can do is the best you can for these animals and accept the things that are out of your hands.

-adam