The medical term for bleeding under skin is "petechiae," flat, painless, pinhead-sized spots of red or purple, caused by local bleeding from small blood vessels. Petechiae appear and fade in crops and can group to form larger, nonelevated patches called ecchymoses. Ecchymoses may look like purple, blue, or yellowish-green bruises that vary in size and shape. They commonly occur as a result of trauma. Both petechiae and ecchymoses are seen in animals with bleeding disorders. Most bleeding disorders are caused by destruction of platelets in the circulating blood (thrombocytopenia). Petechiae have many causes, including external and internal toxins, bacterial infections, autoimmune diseases, bone marrow cancers, drugs, and vaccines. Other bleeding disorders are caused by inflammation of the blood vessel walls (vasculitis). Vasculitis can affect any organ, although in cats it is mainly seen as ulceration of the footpads, ears and lips. In many cases it is a transient condition that can progress to a chronic form, sometimes termed as vasculopathy, suggesting underlying diseases or disorders.
Bleeding under skin can also be caused by rodenticide poisoning. Most rodenticide preparations contain anticoagulant agents which disrupt the normal blood clotting process in rodents and cause lethal hemorrhage. In case of rodenticide poisoning, cats may bleed from any orifices of the body, or bleeding may occur within or underneath the skin. Many cats develop internal bleeding that manifests as difficulty breathing and abdominal swelling. Treatment of anticoagulant rodenticide poisoning involves the administration of vitamin K.