you need to get to the er now. with your medical history it is not that important when you started bleeding but rather how much you are bleeding right now. a rapid bleed in the stomach could cause black stools relatively fast. because you are weak and out of breath it would not be wise to delay an immediate medical evaluation.
my stool is jet black i've had this before how long does it take for the stomach to digest and pass blood from an ulcer. i've got a lengthy history of bleeding gi's which has resulted in multiple hospitalizations for a week or more at a time each time requiring transfusions from anywhere to 9+ and sometimes up to 14 units of blood. even though i just noticed it i'm already growing weak and getting out of breath for no apparent reason something that never happened this soon before. so i was wondering if i've been bleeding in my stomach for a long time and it's just now being expelled.