Gi bleeding can be without warning, brisk, & originate anywhere in the gut (from esophagus, stomach, to rectum. Don't hesitate to seek urgent evaluation/care especially if lightheaded, dizzy, weak.
Sources of particular concern: bleeding stomach ulcers, ischemic colitis
, infectious colitis, inflammatory bowel disease
. Get right to a doctor, &consider stopping any non-prescribed NSAID drugs now.