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Congratulations on your 10th year of transplantation
. In order to determine the cause of anemia, more laboratory studies will be required such as iron studies, thyroid panel, B12, folate level etc. your anemia may be multi-factorial and review of all of the above data would be required to determine an exact cause.
Since you have blood in the stool (occult blood based on what you're describing) and you are a recipient of a solid organ maintained on transplant medications, you require a colonoscopy
[if not done recently] and possibly an endoscopy
to delineate the source of the blood.
Your issues can not be graded as serious or not without more information. For example blood in the stool with anemia can represent anything from peptic ulcer disease
to hemorrhoids or diverticular disease
Take care and be well