Cause of Sudden Death of Rednose Pit Bull?

Unfortunately 2 nights ago when I got home from work my heart broke when I found my dog dead in my backyard. I have no idea what has caused his death. He was a very lively and active rednose Pit Bull and was 7 and a half years old. He was stiff and cold when I found him and he appeared to be swollen around the stomach and rib area. I also noticed a small amount of blood coming from his mouth. His eyes were closed, but I took a close look and noticed they looked bloodshot. I noticed he still had half his food left in his bowl and half on the side and he never left food uneaten he always scoffed it down. At first I though he may have chocked while eating, but if that was the case I doubt he would have crawled to the corner of the yard about 20 meters away to die. So I'm just wondering if anyone has any idea what's happened to my boy as the vet has been no help at all and 2 days later I'm still waiting for a call back.