Give it another few days for your pup's stomach to get used to the new ingredients of the dog food and settle in. We have 3 pit pups ourselves - all about 10months to a little over a year old and have used at least 3 or 4 different brands of dry dog food... we started off with PetSmart's own store branded kibble that was promoted to us by their in-house vet. After a month or so, we went with a family member's advice and we switch to Nutro Ultra as his pittie was doing well with that food... then after joining the GP forum, we learned more about quality dog food and switched to TOTW, but our pups had a real bad case of the runs which lasted for about a month and after a visit or 2 to our vet. We were then advised by our vet to speak to the local feed store owner and he recommended DVP Natural Balance Limited Ingredients. It is all trial and error to be honest with you, as each of my pups have experienced gastrointestinal issues to some extent. Our boy, Cujo seemed to have the worst experiencesm as he has such a sensitive stomach and any change to his diet would result in a period of soft stool to runs. So now, we have stayed with DVP's Natual Balance Limited Ingredients for all three of our pups and since it comes in a variety of flavors, we rotate it everytime we buy a new bag from Chicken to Bison to Lamb to Fish. The Limited Ingredients version has worked very well with my pups as the "limited ingredients" of the food keeps their stool firm.
If you have any other questions, definitely let me know.