What Causes Frequent Discomfort In Stomach?

Over the last few months, I have been awakened by a feeling in my stomach like I am on a roller coaster and just dropped down a big hill. It only lasts a few seconds but is enough to awaken me briefly. I have only noticed it while sleeping on my back. It usually is only a once in a while thing, but last night it happened several times. What can be going on?