Could be parasites (like maybe coccidia which needs antibiotics to treat), internal blockage or damage from something they ate, bacteria or viral infection and if only one vaccine and a puppy it could still be parvo (and if soon enough your vet can prescribe something like Tamiflu which is seeing good results and a faster recovery in many cases). They aren't considered protected until after the last puppy shots as titer levels are often too high or to low for the vaccine to be effective so we just have a schedule that is our best chance of hitting our window of opportunity and parvo often takes later in the sets towards the final ones. You can ask your vet about titer levels, why pups need so many sets of shots and the window of opportunity for more details probably. They will have a better idea of what it may be once they are able to examine your pup and speak to you. You should see your vet today if you can. Hope it's minor and your baby is okay.
www.merckvetmanual.com
www.cascadesummitvets.com www.dogfoodanalysis.com
www.bornfreeusa.org
www.merckvetmanual.com
www.cascadesummitvets.com www.dogfoodanalysis.com
www.bornfreeusa.org