Do you have that reaction to ANYTHING else? Like broccoli? You could have an allergy to sulphur. I am by know means knowledgeable about food allergies, but I do know garlic is high in sulphur, and that allergies to sulphur can be very severe.
Look up "sulphur allergy symptoms", and be sure not to confuse it with sulphA wich is far more common and NOT the same thing. Some are allergic to both, but if I remember correctly, that's not very common.
I'm going by a friend who had an allergy to sulphur - it sent her to the ER several times before they caught it because foods containing sulphur vary far more than say, dairy or peanuts. Ie, I think sour cream is also on the sulphur no-no list.
Good luck & be careful what you eat until it's identified (my friends was misdiagnosed as food poisoning on at least 2 occasions, and having no clue of the culprit, kept eating sulphur. Most often she'd get headaches & stomach aches that she never connected to food, but if she ate a LOT of it, she was severely ill with symptoms much like yours)
I would cut out garlic for awhile, make sure it is not in anything you eat.
You should also see a doctor if you continue to have blood in your stools. Maybe the garlic is causing it as it is kind of rough for some folks to digest, but, whatever the reason, blood in the stools need to be checked as soon as possible.
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