Yes, post nasal drip can absolutely cause stomach issues.

When excess mucus drains down the back of your throat, it is swallowed and enters your digestive tract. Because the stomach is not designed to process large amounts of phlegm, this drainage can lead to several specific stomach issues.

Common Stomach Symptoms

• Nausea and Vomiting: The swallowed mucus can directly irritate your stomach lining, and constantly swallowing it can trigger your gag reflex. 
• Bloating and Gas: When you frequently swallow to clear the mucus from the back of your throat, you often swallow excess air (aerophagia), leading to gas and abdominal bloating. 
• Stomach Pain and Indigestion:
  Excess mucus accumulation can cause an upset stomach and mild abdominal pain.