have you fed anything differently recently? If not i would take a stool in tomorrow for a fecal with a lab due to being a pup it can be a number of things and since it has happened for a few days and blood in it I would want it checked
keep in mind diarrhea is much more dehydrating than vomitting
It is good she is acting normal though but I still would have stool checked tomorrow
here is some info - it could be no big deal but due to age there are some things I would want to rule out
Causes, Diagnosis and Treatment of Diarrhea in Dogs