With plavix or any antiplatelet medication, controlling bleeding can/will be difficult and requires hemostastic measures and blood clot considerations and treatment. There is a small percentage of people that should not be on plavix and apparently your mother is so classified.
An enlarged heart for an older person is not unexpected and other conditions and circumstances would determine seriousness of an enlarged heart. Generally, with an enlarged heart, the amount of blood pumped into circulation is reduced, and there will be some shortness of breath with exertion and muscle fatique that limits an older person's activity. If it becomes worse, the same symptoms occur without exertion.