We generally expect to see a vaccine/medication/blood transfusion reactions withing 15 minutes of administration (to humans). Bloody stool doesn’t sound like a drug reaction to me, it sounds like the dog ate something it shouldn’t have or has an intestinal infection (hence the metronidazole). If something is irritating a dogs GI tract, we expect to see mucous in the stool. This may be the case with worms or as a result of eating inappropriate objects as puppies are prone to doing. vomiting and diarrhea is also a sign of K9 pancreatitis which is pretty common. Hopefully the vet ruled that out immediately with a snap test. It’s hard to have faith in the vet sometimes but I generally try too as they’re generally very intelligent and qualified. the famotidine is for both allergic reactions and for acid reflux type conditions btw.
I put the eating something wrong question the the Vet last year and it was ruled out. I'm usually right there with them when I let them out due to the blasted Yotes around. I don't toss out scraps so that's ruled out and my neighbor is a ways off the dogs don't wonder that far. i can safely rule out anyone tampering with them like poison also. Not many people even know my house exist. It's up off a dead end road and almost a quarter mile off that road.
IIRC pancreatitis is a common issue in Ratty's though.