It has happened to ours also, I think all three of them once or twice has thrown up mustard yellowish slime. I connected it with times where their belly have been empty for some reason, so I always try to give them snacks in between food, doesn't have to be anything fancy, just so they have something in their belly I hope maybe that is what's going on with Winston? Maybe his stomach is sensitive? I had a Jack Russel that I had to feed 4 times a day even when adult for that reason. Vets are sometimes a bit difficult on the phone cos they want you to go there and pay, lol, but maybe if you have a good relationship with yours they can tell you something that will give you peace about it, or if they think you should bring him in.
Hope it's nothing, I know how horrible it is to worry about them I always stress for the smallest thing and I think sometimes I make things up in my mind, lol