Low platelet count manifests itself first as little tiny red dots, much like fingernails would leave if you sctratch one spot too much. Then if platelets drop more, dots will increase up to 2 mm in size. By the time bruises develop, the situation is critical, and internal bleeding on some of the organs is more then likely.
Sounds more like a clotting problem. She should get her blood tested to see if she has low platelets. If you can't clot properly, small bangs and bumps rupture capillaries, and you bleed enough for it to show as a bruise, where normally you'd be fine. Does anyone in her family have a clotting disorder?
Sounds like anemia. She needs to see a doctor because there are several types of anemia - B and iron are only two... Her blood is too thin for some reason.