Your dog is vomiting white mucus what is it?

You ask your vet. He'll be glad to tell youMy dog is coughing and trying to bring up white mucus, very little starts to come up then he stops. He is 5 years old, boarder collie/x. He has been very well up until February when he started to stink, vomit yellow bile, lick everything, his bed, leg ect. I stopped the dog food and have been making him his food since. I make fish, a bit of brown rice, steamed carrots, broccoli and cauliflower, topped with a tablespoon of fish oil every few days. What could this be. He coughs usually while lying around, not while out for a walk. He is a bit over weight and not your typical boarder collie. He could lay around for 10 hours if you let him.