You have developed a bruise over your skin due to injury
Since you had a sudden development of the same, some type of trauma to the skin seems to be the most likely cause of the same.
A bruise develops due to internal tear in blood vessels and bleeding.
Just apply thrombophobe or hirudoid cream over the affected area.
Thrombophob ointment contains Heparin-Benzyl Nicotinate which improves the blood supply and prevents clots. so it will help in faster clearing of the clotted blood.
And you can take oral vitamin C
500 mg twice a day orally for rapid healing.
Hope it helps
Dr Kavita Paul