Unexplained bruises on your arm could be a sign of underlying health issues, including cancer. Leukemia, for instance, can cause easy bruising due to low platelet counts, leading to bleeding under the skin. If bruises appear without injury or last longer than usual, it’s crucial to consult a doctor for further evaluation. Cancer can disrupt the body’s bleeding and clotting system, making bruising more common. Medical assessment is necessary to determine the cause of such symptoms.