My Lady has been a diabetic for over six years. Stress can raise blood sugar in a diabetic dog, but I have never heard of it doing so in a healthy dog. I assume that she fasted before her spay which makes it even more unlikely as it's food that make the blood sugar rise.
While most diabetic dogs are older (Lady was 6.5 when she was diagnosed), juvenile diabetes affects dog under one year old. It is fairly rare and is thought to be genetic.
Dog Owner's Guide: Canine Diabetes Juvenile Onset Diabetes Mellitus (Sugar Diabetes) in Dogs & Puppies
Cushings is also a disease that usually effects much older dogs. It is more difficult to diagnose.