Hello, Thanks for your query. If you are pretty sure that you haven t injured your arm I can think of Spontaneous bruising. A spontaneous bruise, also known as an easy bruise, is one that occurs on its own and the person has no recollection of what caused the bruise. This type of bruise can begin to form as you watch it. It usually begins with the area swelling up and within an hour the bruise begins to appear. Many times the area will be sensitive to the touch, or feel as if you had hit it against a hard surface. For other people, a spontaneous bruise may occur from lightly tapping the skin, a touch that normally doesn t cause a bruise to occur. It has been observed that bruises are often observed more on women than men. Women often complain of bruising on legs, thighs, buttocks and upper arms. Thinner people are more prone to bruising as they lack the fat cushion that helps prevent injury. It may also be due to deficiency of vitamin C, vitamin B12, folic acid and vitamin K I do hope that you have found something helpful and I will be glad to answer any further query. Take care