Gas can occur in plenty of places, not just the stomach area. Trapped gas can even make your back hurt! Cancer doesn't just "develop" immediately and say "Hey I'm here!" If your symptoms come and go, get better, or go away for a time: I doubt it is cancer. Cancer gets progressively worse the longer it is left untreated.
(I don't think PB&Js can give you cancer.) Pop a couple of Tums and see if that helps. I have bad heartburn and acid reflux, so I try to keep a bottle of Tums handy if I know I'm going out to eat or will be eating something I know that will upset my stomach.
What is your dairy intake like? I've had to cut back on milk and cheese significantly because it gave me a floating feeling in my chest and constant burping/stomach pain.
As for the cramping: anxiety makes our muscles and nervous system very tense in preparation for imminent danger (9 times out of 10 we're in no real physical harm, but the mind is a powerful agent of doubt). Take a warm bath, drink some tea, and watch a funny movie. Or play a fast paced video game to take your mind off it.