Hi, Welcome to JustAnswer. I am a licensed veterinarian and I'm here to help you. It does sound like she will whelp in the next few hours. When a dog's temperature drops below 100 degrees fahrenheit (and Bella is at 98.6 degrees fahrenheit), and stays there, then whelping should occur within 24 hours. (Sorry to be speaking in fahrenheit. . .I'm in the States).
If Bella is acting fine otherwise, then I don't believe her temperature drop is due to any illness. It really is not uncommon for a soon to be mom to bury her food for future use.
Thanks for your response! Eating grass can indicate an upset stomach. . .which is also common when they are huge or beginning labor. The fact that she is whimpering could indicate that her uterus is starting it's contractions. If you place your hands on her abdomen, can you feel her uterus contracting?
Once she is obviously laboring, a puppy should be delivered in no longer than 30 minutes. If not, you do need to see veterinary care right away.
Obvious labor is when you can actually see the sides of her abdomen contracting.
What kind of dog is Bella?
So a larger dog with slender head (not a brachycephalic like a pug or bull dog). Her shape and size should lead to an uncomplicated birth.
One other thing you can evaluate is her vulva. You'll want to make sure that no puppies are visible without being delivered quickly (this should also happen naturally).