Hello, Thanks for your query. If you are pretty sure that you haven t injured your calf 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. An occasional spontaneous or easy bruise is generally nothing to be concerned with but if it is occurring often it may be time to see a doctor because bruising may be a symptom of a more serious condition such as a blood-clotting disorder or blood disease. If this is the first time something like that happen you should not worry. I do hope that you have found something helpful and I will be glad to answer any further query. Take care