Don't worry. It's just your stomach acid flowing back into your esophagus
Heartburn happens when your stomach acid flows back into esophagus
It could happen because of a problem with your muscular valve called the lower esophageal sphincter, or your LES.
What caused it?
Well, normally the LES keeps stomach acid right where it should be. When it's working right, the LES opens to allow food into your stomach, then closes again. But if the LES opens too often or doesn't close tightly enough, stomach acid can seep into the esophagus and cause a burning sensation, which is what you are feeling