Could be from the increased hydrochloric acid production in your stomach. remember, anytime a meal is taken in, acid producion increases to aid in the digestion of food. Thus, you may be digesting food a little more rapidly than normal. Very important that you don't eat 3 hours prior to going to bed b/c of this. The lower esophageal sphincter is usually a culprit for acid seeping into the esophagus, causing "heartburn."