Hello, Most of the time patients do not remember when they get hurt and are surprised of bruises in the areas they have hit against something. Very likely this is a bruise due to local trauma, even though you don t remember the trauma. Being on the leg is a reason why it is likely related to trauma as bruises on the legs are very common. On the other hand if you have other big bruises coming up in your body or extremities or you have gum bleeding or nose bleeding other other bleeding in your body like blood in urine or with feces then it would be safer to see a doctor and have some tests to rule out problems with coagulation or the bleeding time. I hope I have answered to your query. Let me know if I can assist you further. Kind regards, Antoneta Zotaj General & Family Physician