I'm a healthy 21 year old male but for the last couple of months I've been experiencing a few unusual symptoms: - Immediately after eating a meal, I get this dull aching pain in my stomach that radiates to my left back. (I get relieved from this pain when I lay down) - I get nauseated and constantly feel like I have to burp. (The nausea is moderate but I've never vomited) - I get very satiated and full, even if I ate very little of the food. - I pass hard and pellet-like stool (Not sure if this is related to my stomach symptoms). All the symptoms I have last for about 2-3 hours after the meals, then I feel better again. What's causing this? Could it be gastritis?