new puppy has diarrhea

This question is actually for my mom, since I have posted questions here before and she's new to this message board thing!
My mom bought a 7 week old chocolate lab about 3 weeks ago. She had her first vet appointment last week and Sophie's a healthy pup (no worms etc). When they first got her, the breeder had her on a Purina diet, but I soon talked my mom into switching to something healthier and she now uses what I use, Natural Balance. She switched the food slowly over time as recommened and she's eating and drinking fine. The issue at hand is diarrhea and/or loose stool and seems to have frequent bowel movements. This was happening before the food switch and is continuing to happen still. Could it be a reaction/allergy to the food, nervous from being taken away from mom and siblings, scared from being left alone all day in a crate? And ideas and thoughts on how to help Sophie?
Ashly Burmeister

About a week after I posted this question, mom noticed a few pockets of blood in Sophie's stool. The vet put Sophie on a dewormer, even though her previous checkup showed no signs of worms. Not sure if it's helping at this point. I think the diarrhea is still occuring. Could an intestinal parasite be causing the diarrhea and bloody stool? Would the dewormer rid a possible parasite?