Soft Stools Help Please!

Recently, my Brownie has been having lots of soft stool and just going to do a lot of business. I had suspected worms but that is not the case. Our Vet did a fecal and did not find anything. She said perhaps it's from overeating. I am not sure if that answer satisfied my concerns. If anyone has any suggestions, please comment and help us out! Appreciate your help. :)