Your problem is likely to be related to the digestive system according to the symptoms which include belching. The mucus you vomited may be the secretion from the oral cavity or pharynx. However, if the mucus tastes sour or bitter, it may be secretion from the stomach due to reflux. Gastritis (inflammation of stomach) or peptic ulcer at the stomach or duodenum among other diseases can cause these symptoms. Besides, smoking, eating too quickly or irregular meal times can also lead to indigestion. You should see if quitting smoking and enjoying your meals slowly at regular hours help. If your symptoms stay the same or worsen, you should seek medical advice immediately.