I understand your concerns but there s no reason to be worried. The symptom that you have described is probably suggestive of indigestion. However it can also be due to various reasons like constipation, food poisoning, and gastro esophageal reflux disease. Since you have mentioned that the food remains in the stomach and you feel a rotting smell it could be mainly due to indigestion. Here patients present with bloating sensation, abdominal pain, belching, acidic taste, and nausea. This can be best diagnosed by an endoscopy where the doctor inserts a flexible tube attached with a camera inside the mouth and it traces the stomach lining. In case there are any abnormalities, then a biopsy is taken. I would suggest you to get a complete blood test, ECG, endoscopy and consult your doctor with these reports for further treatment.