How heavy is "heavy"? It sounds like ecchymosis if you are referring to more of a bruise like stain underneath the skin. Your description doesn't tell us much. "accumulation of blood" doesn't tell us if you are bleeding or just bruising in the area.
Is this the only area this ever happens in? Are you on any medications? Do you take asprin? Are you a hemophiliac?
The cause of this can range from "normal" to the presentation of a disorder.