If it is a vomit ,then usually the yellow color in vomit are due to the chemical, bile, which is produced by the liver. Normally you never see it in the stomach because normal eating keeps it moving down the GI tract.
The presence of yellow saliva early in the morning may result from the buildup of bacteria on the teeth, tongue or other tissues in the mouth. This often results in halitosis, or bad breath, and can increase the likelihood of tooth decay.
Bacteria in the mouth can usually be controlled by good oral hygiene practices, such as brushing and flossing regularly. Brushing or scraping the tongue is especially important for halitosis sufferers.
If these measures are not sufficient, a dentist may be able to provide other suggestions for getting rid of the bacteria that cause halitosis and yellow saliva.
A cold, sinus infection or other respiratory illness, however, often causes mucus to turn yellow or green. Either viral or bacterial illnesses can result in yellow mucus. Some of these conditions clear up on their own, but if abnormal saliva and mucus remain for more than a few days or are accompanied by severe discomfort,you should check with your doctor
Acid reflux may also cause yellow saliva WHICH I SUSPECT IN YOUR CASE since you say i feel something coming up causing discomfort, especially in the morning.
This is a condition in which the digestive juices found in the stomach are regurgitated into the esophagus. When the patient is lying down, these juices may make their way into the mouth, resulting in yellow or brown saliva.
If the yellow saliva is accompanied by heartburn or nausea, it is likely due to acid reflux. In addition to the discomfort of acid reflux, the presence of acid in the mouth may lead to tooth decay or other oral health problems
Most people have an occasional bout of reflux, but if it happens more than twice a week, doctors call it GERD. If left untreated, GERD can cause permanent damage to the esophagus, causing a narrowing of the esophagus or an open wound.
Concerning the link to the protein shake use this is as follows :
Protein shakes are usually healthy to drink. But there are many reasons why they might give you heartburn.
-and carbs are often to blame.
>You may want to avoid a shake with dairy products....the dairy slows down the sphinter at the lower end of ur espohagus allowing reflux to occur.
>try another brand or switch to a simpler kind of protein shake.
>there are some weight-gainer protein shakes that are sold. These are mainly targeted to young males who want to bulk up quickly. These are sometimes a bad idea.
>Weight-gainers contain fat and carbohydrates,not just protein. These tend to be tastier than the pure protein shakes. You could switch to whey protein isolate shakes.
>Focus on food better
>if you are lifting, for a weight lifter, the excess pressure on the abdomen is proven to also cause heartburn.
Therefore ,If you try a protein drink and it seems to give you a stomachache, that doesn’t mean all other protein supplements will affect you in the same way. Before writing them off, try a drink made with a different source. If you drank a casein or whey shake, for example, go for a powder made with a plant-based protein like soy, brown rice, hemp or pea. See your doctor before you begin using any protein supplement, especially if you have food allergies.