Menstrual Bleeding: This is a common cause of heavy bleeding in females, especially if they are of reproductive age. Lightheadedness can accompany heavy menstrual bleeding due to significant blood loss leading to decreased blood volume and subsequently reduced blood pressure.

Gastrointestinal Bleeding: Bleeding from the gastrointestinal tract (e.g., ulcers, varices) can lead to significant blood loss, manifesting as melena (black, tarry stools) or hematochezia (red blood per rectum), and can cause lightheadedness due to hypovolemia.
Postpartum Hemorrhage: In the context of recent childbirth, heavy bleeding can occur due to uterine atony or other complications, leading to lightheadedness from blood loss.
Trauma: External or internal injuries from accidents can cause significant bleeding, leading to lightheadedness or even shock.

Ectopic Pregnancy: Although less common, an ectopic pregnancy can cause significant internal bleeding leading to lightheadedness and even shock if not promptly diagnosed and treated.
Ruptured Aneurysm or Arteriovenous Malformation (AVM): These conditions can lead to sudden, severe internal bleeding, causing lightheadedness and potentially life-threatening outcomes if not immediately addressed.
Placenta Previa: In pregnant women, placenta previa can cause significant vaginal bleeding, leading to lightheadedness and requiring urgent medical attention.

Von Willebrand Disease: A bleeding disorder that can cause heavy menstrual bleeding or bleeding after trauma, leading to lightheadedness due to significant blood loss.
Hemophilia: Though more common in males, hemophilia can cause significant bleeding episodes, potentially leading to lightheadedness if not managed properly.
Coagulopathy: Conditions affecting blood clotting can lead to excessive bleeding from minor injuries or spontaneously, causing lightheadedness among other symptoms.