"And My Stomach's ... Burning"

I'm a healthy, active, intelligent, 20-year-old female; I think we can pretty much agree on those points. Yet today marks the third day in a row that I've been crippled by intense stomach pain. This actually used to happen to me all the time in high school, whenever I was stressed and didn't eat well enough or regularly enough. It didn't happen at all my freshman year of college, and I thought maybe I was cured or had grown out of whatever. I think I had a mild case over the summer, and in the fall there were a couple nights I fell victim to the pain. I remember one time this semester it hurt so badly that I literally cried out in pain when a friend of mine made me laugh. And now this. I wish I knew what to do.