The food pipe is  6 feet long and stomach is a part of it which retain the food for about four hours so that food can be churned,  mixed with juices,  bile made in liver and sent towards stomach by tiny pipes called bile duct.

Bile is bitter in taste.

Stomach does have acid to make the food soft.

Stomach has two valves on either side so that the food goes one way Only and does not go backwards.

The other end of stomach sends softened food towards small intestine where the essential nutrients are taken up by blood and waste goes forward towards large intestine. 

With age the valves of stomach get weak.

It can happen otherwise also due to many reasons like erratic eating habits. 

When the valve on the upper end is not working well it throws food towards the mouth via esophagus near our respiratory pipe. 

The acid plus bile meant to soften food come in contact with delicate tissues of mouth called mucosa,  can cause ulcers in the mouth or give a bitter taste in the mouth. 

How to correct it? 

Eat light food that doesn't have to stay for long in stomach 

Drink lots of water preferably warm and mixed with honey or ORS variety 

Plain water does cause distension 

Too sweet water causes diarrhoea 

Too much of salt is not good for anyone. 

Drink some warm fluids like green tea,  clear soups,  lemon water or just plain ORS in a liter of water

Eat two hours before sleep.

Drink a glass of warm water with a spoon of honey plus juice of fresh lemon as first meal.of the day.

Morning tea can be had half an hour later 

Drink a glass of warm water as last meal of the day.

GERD is a gastro esophageal intestinal Reflux Disease ,caused by food and cured by food only minus Medicines.

Eat well , eat health and get healthy habits of drinking water in between meals , and not munching snacks but one can take an apple/banana/biscuits minus fats etc. 

These work like Cotton and absorb the extra acids/ bile generated and collected under 2 conditions

fasting or

feasting.