Mild discomfort/pain when press against my stomach lining. Other symp: gas, burping, bloating feeling, weight loss. 24yrs old male. What could cause?

Gastritis? You could have gastritis, which is an irritation of the stomach lining. If you are losing weight, you should see your doctor to make sure it isn't any more serious.
Lots of things. The most common at your age would be indigestion, eating/drinking too many gassy foods or drinks (especially beer). If you do drink more than one beer a day, though, it could be more serious like an ulcer or liver disease. Cut out all gassy foods, drinks, gum, and alcohol and try an antacid with simethicone. See doc if not better after a week or two.