Fizz makes you burp. A full stomach needs to empty. When its air that fills you up, esp. If soda is caffeinated, the lower esophageal sphincter relaxes & allows you to burp. In so doing, other stomach contents including acid may get burped up as well. Read other healthtap answers regarding diet & behavior to minimize heartburn. If you belch a lot cut back on caffeinated & fizzy drinks, coffee, & avoid air swallowing.