My White Boxer, Rex, is about 2.5 y/o. I rescued him about a year ago and have been having gi issues since. I've tried several foods (currently on Pulsar grain free fish formula) and I also mix in fiber a supplement & probiotics. We typically walk about 2 miles in the morning. His poo is great in the morning when we first start off! However, after about a mile, it gets much worse, very loose, sometimes liquid- no signs of blood, weird colors, or heavy mucus. He is very energetic, great appetite, no vomitting, no accidents in the house. He's had fecal tests done & they've come back negative. I'm wondering if it's possible that he just gets excited & his system gets worked up??? I hate to put him through unnecessary tests at the vet if it could be a simple solution! Any insight you can offer would be GREATLY appreciated! Thank you!!!!