I'd vote for the worming tablet. Frosty only gets those once every three months. Every month, she gets the flea stuff with no adverse reaction (Frontline anyway) apart from disliking the initial cold blast to the back of the neck. The worming tablet - always gets weird poo for the next 12 to 24 hours.
you could try keeping a diary of what you've been feeding, when the wormers are, and the flea treatment and the vaccinations etc and how your dog is each day - to see if there are any patterns.