Anemia: This condition is characterized by a decrease in the number of red blood cells or the amount of hemoglobin in the blood. It can cause lightheadedness due to reduced oxygen delivery to tissues, palpitations as the heart tries to compensate for the lack of oxygen, and bruising due to possible platelet or clotting factor deficiencies associated with certain types of anemia.

Bleeding Disorder: Conditions such as thrombocytopenia (low platelet count) or clotting factor deficiencies can lead to bruising and, if severe, can cause significant blood loss leading to lightheadedness and palpitations.
Cardiac Arrhythmias: Abnormal heart rhythms can cause palpitations and, if they significantly impair cardiac output, can also lead to lightheadedness. Bruising might be less directly related but could occur if there's associated trauma or anticoagulation therapy.
Hypovolemia: Dehydration or significant blood loss can lead to hypovolemia, causing lightheadedness and palpitations due to decreased blood volume. Bruising could be present if the cause of hypovolemia is related to trauma or bleeding.

Sepsis: Although less common, sepsis can present with lightheadedness due to hypotension, palpitations from the stress response, and bruising if there's associated disseminated intravascular coagulation (DIC). Missing this diagnosis can be fatal.
Cardiac Tamponade: A life-threatening condition where fluid accumulates in the sac around the heart, impairing its ability to pump. It can cause lightheadedness, palpitations, and potentially bruising if there's associated trauma or anticoagulation.
Pulmonary Embolism: A blood clot in the lungs can cause sudden onset of symptoms including lightheadedness and palpitations due to the blockage of blood flow. Bruising might be present at the site of a deep vein thrombosis (DVT), the common precursor to a pulmonary embolism.

Vasculitis: Inflammation of the blood vessels can lead to bruising and, in severe cases, organ damage that might cause lightheadedness and palpitations.
Thrombotic Thrombocytopenic Purpura (TTP): A rare blood disorder characterized by the formation of blood clots in small blood vessels throughout the body, which can lead to bruising, and potentially lightheadedness and palpitations if the condition significantly affects blood flow or leads to anemia.
Ehlers-Danlos Syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. Although rare, it could potentially lead to easy bruising and, in severe forms, affect the cardiovascular system, leading to palpitations and lightheadedness.