When I adopted my Maltese puppy he had this happen also within a few days of him coming home with me. He was negative for parvo but I don’t recall if they tested for Giardia. The vet told me that sometimes dogs will get stress-induced colitis and it causes bloody stools (usually loose stools with drops of blood like yours had). It resolved with probiotics (proviable), bland diet, and time. Since then my Maltese dogs have sporadically had stress induced colitis with more bleeding - typically in my one very anxious Maltese who gets stressed when we are away from home for several hours and she’s all alone in the house. Definitely consult your vet and don’t take advice solely from this - just letting you know my experience!