I have sick for the past few months?

I'm wonder on whats wrong with me because some days I feel okay and seem to be eating fine. But on the days when i'm not well i barely eat anything!! About 10 minutes ago I threw up my dinner and sat at the toilet for about a minute after vomiting and then I started to throw up blood pretty dark colored. I've been to the doctors and had my stomach scan at the hospital. They haven't found anything, but I seem to be loosing quite a lot of weight. I lost almost 20 kg's over the past 4 to 5 months. My doctor is now sending me to the hospital but the thing that is annoying me the most is that I don't know what it is and neither do the doctors.