Doctor visit

Friday's doctor visit did not go as I hoped it would. They did not find a bleeding ulcer. So where did the blood come from? It was black so that means that it was in my stomach at one point for my body to digest it. The doctor is concerned because on Sunday morning my hemoglobin was at about 14. On Friday it was at 8.5, at 8 you need a blood transfusion. Where is the blood coming from--we do not know. I go back on Monday for another blood test.