did you actually get a diagnosis of giardia? in my experience, most dogs develop an immunity to it- it really only seems to affect puppies or dogs with immune/health issues, regardless of their exposure.
Do you have lots of migrating water fowl at this time of year? we do, and every year, early spring, my dogs manage to eat some water fowl poop and that gives them a brief bout of diarrhea.
Do you have plants coming up they might feast on, or algae blooms starting up this time of year? that might cause a seasonal bout of diarrhea.
And there are viral causes of diarrhea- last year, in my area, something went around the dog population and caused diarrhea. It was hitting the agility dogs in December-ish, and had moved to the general pet dog population by Feb.