It's more than likely the arrow slid along the ribs between the shoulder and the chest wall, maybe hanging up in the sternum alittle. If you get an arrow between the shoulder and the chest wall the action of the shouder in running tends to generate some bubbles, not a lot, but a few. It's also possible you just knicked a lung and it quit bleeding. Deer are tough if you put it in the wrong place. I've just seen too many, "He was angled towards me" post to ever consider the shot unless it's point blank slam dunk and the damn thing is asleep. My new bowhunter/student did a shot like that last week. I had told him, he took the course, I reinforced on the range... and he still thought he was better than that. His first words to me, " I know... I screwed up... you told me... I learned a valuable lesson". I bet he won't do it again,