It seems to me that what a lot of you are experiencing could be simple heart burn, to a possibly a gastric ulcer.
with heart burn aka GERD, when you eat, proton pumps in your stomach start producing hydrochloric acid to break down the food. If you have an interruption of the mucous membrane of your stomach (ie. Heliobacter pylori infection) the acid starts to burn your stomach, which is usually covered by the membrane. This would give a burning/stabbing pain in the middle of your upper abdomen, right below or behind the bottom of your sternum. The second issue is GERD (gastro esophageal reflux disease). In normal stomach function, once the food (bolus) enters the stomach and HCL is secreted, the stomach starts mechanical breakdown, it literally moves back and forth and over on itself grind the food into a much. during this process the esophageal sphincter closes to stop the acid from going into the esophagus. In GERD, the sphincter is weak for some reason and cannot always close all the way. So the acid is pushed up into the esophagus casing the familiar burning pain and burping in the throat.
With both of these problems, pain can radiate....for instance those of you that eat meals and feel pain in your neck, shoulders and such is most likely due to the Vagus. The Vagus is one of your Cranial nerves and it runs from the base of the brain all the way down into your abdomen and is sensitory and parasympathetic. When you eat and start to exercise or what not, this can stimulate/irritate the vagus causing referred pain to your neck, chest and arms. The parasympathetic function can cause decreased heart rate and blood pressure leading to dizzyness and fatigue. The parasympathetic function tells the body to send blood to the stomach and intestines, starting digestion. When you exercise after eating (mostly with current stomach problems aforementioned) the normal response is to stop parasympathetic functions and start sympathetic functions which decreases blood flow to stomach and intestines, stopping digestion in order to increase blood flow to muscles and hear and other organs needed in the fight or flight response. if the vagus is stimulated from a stomach problem such as GERD or ulcer, and you start to exercise, the vagus overpowers the sympathetic functions and starts digestion and increases blood flow. when this happens with exercise, you have nausea, vomiting, pressure and fullness in your stomach. this is why sitting down helps decrease the pain
to treat this condition simply do the following:
decrease carbonated beverages, hot/spicy foods, eat smaller portions in a sitting (if need be, eat more often, just with smaller portions), dont lay down after eating, raise the head of the bed 6 inches, drink milk (a natural buffer), eat antacids (malox, tums, rolaids etc), or take OTC zantac or prolosec...always consult your doctor before starting a medication. Your doctor can also give you Rx strength proton pump inhibitors

all of these conditions and symptoms should be treated and evaluated by your doctor to make a definitive diagnosis...due to the fact that these symptoms can rarely in younger healthy adults be caused by worse conditions such as: heart attack, bleeding ulcer, abdominal aneurysm, gall stones, pancreatitis etc.

Hope this answers some of you questions and concerns....just see your doctor