"I use one pill per liter and usually wait 15 minutes to 20 minutes before drinking. I also drop a elixir electrolyte tab in at the same time. If the water is really cold, i wait 30 minuutes."
What temperature was the water? It's quite possible that your wait time for protozoan cysts such as giardia may not be sufficient. Here's a table from the EPA for chlorine dioxide tablets:
Here's the gist of the table (also found in many other places, such as the Katadyn website and Wilderness First Aid texts):
Cysts Clear water, 20*C [68*F]: 30 minutes // Turbid water, 4*C [39*F]: 4 hours
It would be nice if the EPA would furnish a few intermediate numbers for estimating contact time for conditions between the two! For those of us who backpack in the high mountains, water from springs is likely to be in the 30s F but is usually clear. It's obvious that the 4 hour wait time on the package is the absolute worst case scenario. I once asked Katadyn if they had any intermediate numbers, and they said no, implying, although not actually saying, that the EPA wouldn't let them release such numbers anyway. This leaves us all up a creek (bad pun, I know) on contact time, unless we want always to assume that worst scenario!
Anyway, it is possible that your stream water was cold enough to require a longer contact time, since it was probably colder than 68*F.
It's also possible that the electrolyte tablet might reduce the effectiveness of the chlorine dioxide tablet because it in effect increases the turbidity of the water. You might consider waiting to add the electrolyte tablet until the chlorine dioxide tablet has been in the water for the appropriate treatment time (whatever that is, but probably longer than your 20-30 minutes). I'm inferring (i.e. slightly educated guess) on this one, though.
Without sampling your water source specifically for giardia (normal water tests are for coliform bacteria), specifically at the time you ingested it, we'll never know for sure!
As for the dog outbreaks--we'll probably never know, either. However, if you've ever watched two dogs greet each other, you can easily guess one possible contamination source! In my dog's case, the backcountry exposure was either 4 weeks or 3-4 days before the symptoms, neither of which fits the incubation period. However, he was at the beach, where he has a habit of lapping ocean water, about 10 days prior to symptoms. He also got acquainted with quite a few dogs during that period. Again, we'll never know for sure.
I do assume that my backcountry water sources may be contaminated, so I filter (heavier but faster and better tasting). You never know what's upstream--I've found both live and dead animals, several different times. Even if only 20% of my sources (generally lakes and streams rather than springs) are contaminated, that's more than I want to risk.
Maybe a dog with giardiasis was in the water upstream of where you got your water?? Now that's really in the realm of pure speculation!