I get blood in stool after drinking, why?

Go see your doctor: Blood in the stool is often a serious problem and msut be investigated as the cause can be simple as hemorrhoids or something very serious like cancer of the rectum or large intestine. So do not ignore it. Alchol itself is unlikely to be a direct cause, yet it can cause gastritis or stomach ulcers which tend to bleed. So better find out what is wrong before you get into a swerious or lifethreaten.