Nothing happens without a reason. If the reason is not obvious, it does not mean that it is not, just did not pay attention to it. The reason for bad behavior is always, and, as a rule, not one.
Pregnancy of a dog is the period of development of a fertilized egg in the future puppy, up to the moment of its birth. For the animal organism, this is always stress and during this period chronic diseases can become aggravated or new health problems may arise.