Poor Cosmo!

We seem to have gotten Cosmo's peeing under control...but we are in our second bout of diarrhea. Starting last night, his stool was loose (not completely watery) and more foul smelling than usual. This morning it is the same consistency but there was a little mucus and a drop or two of blood. Cosmo got his second round of vaccinations yesterday- can that cause stomach/intestinal upset?

He's been very sensitive in regards to dietary changes in the past month, so we have been trying to very slowly switch him over to a better puppy food (his previous owners fed him Purina puppy chow, and thats got way too much filler). His energy levels are great, very active and plays with our older Lab. He's got a great appetite, is gaining weight fine and is generally bright eyed and happy. The Vet gave him two thumbs up at his checkup yesterday.

Is diarrhea like this common in puppies his age ( 3 months next week) , and is there anything I can do to ease the symptoms here at home? The last time he had this the Vet gave him some antibiotics and a little Kaopectate and he was fine a few days later. His stool tested negative for Giardia and Coccidia.