Pain under the left shoulder blade is usually a referred pain from abdomen like stomach ulcers, acidity, pancreatitis, or ectopic pregnancy or from the chest like in heart attack, pericarditis, pleuritis, pneumonia
, pulmonary embolus, and
aortic dissection. All the causes except acidity can be serious, and need to be ruled out immediately. Howevere, most likely, you could be having a strong gastric reflux or hiatal hernia—both can cause popping sound with fluid coming up.