Sever Upper Stomach Cramps

Approx every 3 mths, I have "episodes" characterixed by severe stomach cramps in the center of my upper abdomen.  The cramping usually lasts about 24 hours & is accompanied by nausea & vomiting.  Diarrhea, etc is not a factor.  At the onset it seems as if my body is unable to digest food & it sits in there all day until I throw up.  I do suffer from acid reflux & take Priolsec, but these episodes are not connected to any flare ups or presence of acid.

My doctor is having me do an abdominal ultrasound, but says he is not convinced that it is my gallbladder because the pain is not specific to the right side.

What else could cause such severe pain & cramping only in my upper stomach?