I adopted a dog from my local shelter. He had all his shot before he came home. He has been coughing, it sounds like when they suck a hair or fuzzy up their nose. He does it more at night (he sleeps on has back) and in the morning when he first gets up. Sometimes he tries to gag it up. Nothing has been coming up until this morning. It was greenish-yellow. It was was like very thick nasal discharge. He is eating fine & playing non stop. Could he have a cold??