Have dizzy spells, feel weak. Had a spell where I fainted due to loss of blood. Doctors couldn't find the cause of the bleeding?

Need evaluation. Loss of blood implies bleeding somewhere-likeliest sources are gut, urinary system, gyn; less common can be lungs, or behind abdomen (retroperitoneal). Other causes for low blood count can include bone marrow issues. Need thorough and systematic evaluation soon. May need colonoscopy, upper endoscopy, capsule endo (small bowel), cystoscopy (bladder), ct, bone marrow exam, or other. See your doc.
More work up. Did your doc check urine culture, blood work for kidney function / CBC with diff. If needed bone marrow biopsy - heme/ onc. Consult.