Loose Stool after exercise or excitement ?

Question
Hi,  Our 7 month old chocolate female lab puppy has very loose stool but only after running around the yard or when excited, like going to puppy class.  At first I thought it was the food, Bil-Jac Large Breed Puppy food, what the breeder used and recommended.  I did some research online and decided to change to Chicken Soup for Puppy Lovers for Large breed.  I did the change gradually over a couple of weeks but the problem still continues.  She has normal formed stool before any yard play or excitement, then it gets very loose.  She has been spayed at 5 1/2 months and vet dewormed before that and the recent stool check was negative. She has had all the standard puppy shots plus Lepto and is on Heartguard Plus.  Have you ever experienced anything like this?  Do you have any suggestions. My vet hasen't been able to help.
Thank you,  Tom M.

Answer
I don't have any good ideas.  A second opinion from another vet would be a good idea.  Over feeding can lead to loose stools too.  Your dog definitely should be narrower at the waist than the hips and chest. You should be able to easily feel the ribs, but not see them. Each dog is different. Standard recommendations are a good place to start, but each dog must have its food and exercise adjusted to its individual needs. Here is a link to a good illustrated guide, http://www.longliveyourdog.com/twoplus/RateYourDog.aspx

Early switching to an adult chow for the larger breeds, slows growth and helps develop sturdier joints.  Be careful of the Chicken Soup.  some adult chows have over 1.5% actual calcium, not minimum.  The case for premium foods is weak.  It is all based on speculation.  I have seen very successful large programs feeding Pro plan, Iams, and other common brands.