This sounds like either an allergic or an idiosyncratic response to garlic. I suggest that you cook the garlic well, if you are not already doing so, and in smaller amounts If that doesn’t work, you may just have to eliminate garlic from your diet. There is, however, the possibility that your intestines are reacting to one or more other foods or spices, commonly used in the preparation foods or recipes that also call for the use of garlic.
Bloody diarrhea is a worrisome sign and needs to be fully investigated, regardless of whether it is or is not associated with the ingestion of garlic. There are many causes of bloody diarrhea, none of which should be ignored. Hold on the garlic for the time being and make an appointment with your doctor to determine the cause of the bleeding. Do not simply assume that it is secondary to garlic.
Good luck