Well, chances are you don't. I went through a time when I was absolutely sure I had some kind of tumor in my stomach and that it might even be cancerous. All the symptoms were there. A few of the risk factors were, too. For a long time I refused to the upper endoscopy because I was afraid at what they'd find (and even more afraid of the treatment). I was literally hoping it would matasticize, spread to my brain, and kill me before I knew what hit me. Irrational thinking.
But I didn't have it. Other than my brain getting out of control, I'm as healthy as a horse.
Definitely check it out if you feel it's necessary, but try some of the relaxation techniques on here in the meantime. Write a lot. You'll be okay.
My Brain: My friend, My enemy: A blog to chronicle my attempt to recover from anxiety/panic disorderanxietypanicdisorder.blogspot.com/