How do I know exactly what is internal bleeding?

Many things. There are many causes of internal bleeding. Ulcers of the stomach/duodenum, major artery rupture, crohn's disease, small bowel tumors, colon cancer, colon infections, ulcerative colitis, vascular lesions of the colon, liver cirrhosis with enlarged esophageal veins. Trauma from car accident, gun or knife injury. Symptoms may be none with mild anemia, to severe weakness and low blood pressure.