Why does my stomach hurt?

My stomach has been hurting off and on for the last two weeks. The first day, I woke up and my entire stomach area and my back hurt really badly (from the bottom of my ribs to my lower abdomen). I had some water and threw it up. But my stomach was still messed up. Ever since then, I have had sharp pains all over my stomach area. But the pains come and go. It hurts so bad, but then they disappear for a while. I have been getting a sour stomach for the past week and I dont know what it is.