It is not unusual for your dog’s insulin requirements to change even after a long period of stability. This is most commonly due to:
weight loss or gain
changes in exercise regime
the presence of other diseases
progesterone in unspayed female dogs. See Spaying diabetic female dogs.
It is important to continue to monitor your diabetic dog's progress - even after months or years of treatment - and consult your veterinarian if there are sudden changes or if anything unusual happens.
You may be asked by your veterinary surgeon to regularly check the glucose (and ketone) concentrations in urine and/or blood samples. This gives an indication of how your dog is doing. Based on these results your veterinarian might decide to do a serial blood glucose curve to determine a new insulin dosage.
You should not change the dose of insulin without first consulting your veterinarian.