Hi there:
This has happened occasionally with my two (now almost 11-year-old) dogs from time to time. Here are some things that seem to be related:
A change in their diet - not just their regular food, but also treats, chewies, and other "extras"
Stress
Illness
Reaction to shots or other medications
Eating strange things they find on the sidewalk before I can get it away from them (arg!)
I'd be wary of the explanation that the dog is just "being bad." Dogs generally want to fit in with their family/pack, and I'm not sure they're ever really vindictive -- as one of the previous posters suggested, dogs do experience anxiety and uncertainty in response to changes in the household.
Dogs also get little intestinal/digestive upsets from time to time (just as we do), and this seems like a more likely explanation, given the circumstances.
If your vet OKs it, it may be safe to administer a little Pepto Bismol (but defninitely check with the vet first). This has been effective for my dogs in the past. Sometimes withholding food for 24 hours gives their digestive system time to recuperate from whatever irritation is bothering them, but again, it's best to consult a vet.
I'm a little confused about why your vet just gave up after checking for worms. Both of my dogs have gotten bacterial infections and other "stomach flu"-like issues that caused similar problems, and she treated them with anti-diarrheal shots and gave them fluids to ensure they don't become dehydrated. It really helped. Maybe you need to explore other vets in your area!