Since this only happens in conjunction with alcohol consumption and/or clonidine, there is apparently something, probably the vasodilating effect of both clonidine and alcohol (which, by the way, should not be taken together) that triggers the bleeding. It is also preceded by abdominal pain, which is significant.
It also appears you take clonidine only intermittently, or at least that's what I infer from your post. Please correct me if I'm wrong. If you're doing this, you need to be reminded that clonidine shouldn't be taken intermittently nor stopped suddenly. One needs either to take it consistently or taper off it.
I would strongly suggest you set up an appointment for a consult with a gastroenterologist, as vasodilation should not, by itself, cause either abdominal pain nor blood in one's stool. There is an underlying cause of this, and the chemicals are simply aggravating the problem, which is not normal. Blood in the stool, even if predictable (based, for instance, on the ingestion of certain innocuous things like an occasional drink or blood pressure medication) should absolutely not cause this. Get it checked and don't wait long. It could be a simple problem or a more serious one, but in any case, blood is supposed to stay inside us, which is why we consider bleeding of any sort a sign of a problem.
Please have this checked out and update us when you have. Follow up as needed in the interim. Best of luck to you.
John Kenyon, EMT, CCT
Non-invasive cardiology tech, Emergency and Critical Care technician, Critical Incident Stress Mgmt. specialist