What Causes Frequent Stomach Pains?

What is wrong with me? For the past couple of months, I have had stomach issues (constant pains in the mornings, nausia, trouble eating, etc); recently, however, the pain in my stomach went from only being in the mornings to all the time. I will eat two bites of a sandwhich, or I will have five grapes and I will be completely full. In addition, it hurts when I eat. The pain usually occurs on the upper part of my stomach, close to the breast plate, but some times there will be pain on the lower-left side of my stomach. Often, I feel the need to puke, but I never do. I'm seventeen years old, about the average height and weight for an Ameican girl. I have never had sexual intercourse, so pregnancy is ruled out. Is there a possibility that I have an ulcer? Or maybe something else?