Perhaps excess acid in the stomach caused by stress, or eating foods which contribute the excess acid like tomatoes, citrus, alcohol, coffee, spicy foods, tea, etc. Have you tried something like Pepcid or Tums--something to neutralize the acid? If you cut down on the culprit foods and the antacids don't help, you need to see a doctor to safeguard against a possible ulcer or acid reflux. Hope you feel better soon!
Soibhan