The cause of reflux lies in the stomach when the stomach acids are forced back up your esophagus and create a burning type pain in your lower chest, the cause of this can be as little to eating the wrong foods or as much as Acid Reflux Disease which you could be diagnosed with at any age, although doctors still arent sure what causes it. Pregnancy, smoking and stomach abnormalities are all common causes of relux. One common cause of acid reflux disease is a stomach abnormality called a 'Hiatal Hernia'. Hiatal Hernia happens when the upper stomach moves above the diaphram allowing access for acid to arise up into the esophagus.
Explain why reflux is referred to as heartburn if it occurs in your stomach
While it does sound dangerous and quite scary at first but heartburn has nothing to do with your heart and is purely called that because it feels as if it is occuring in your heart (or around it) as it occurs jst behind the breastbone in the core of your chest. Although it does hurt and is quite irritating it is perfectly safe for it to occur unless it becomes chronic and appears often when it can lead to a disease called Gastroesophageal Reflux Disease (GERD) which can lead to cancer in the esophagus if left unreated.