I had blood in my stool too and thought it was nothing. It lasted for a month or two before I decided to see a doctor about it. He tested me for lots of things and it took about a year for him to finally figure out I had ulcerative colitis lol. I had other symptoms too, however. I don't drink but I'm guessing that drinking as much as you did where you threw up afterwards can not be good for your body lol.
No, you need to see a gastroenterologist and get evaluated. Dark blood usually indicates an upper GI bleed. Don't bother with your GP this is not his specialty. Poor diet will cause constipation, but not bleeding.