Bruises often go through a cycle of color changes, and you have described a classic pattern of first turning slightly yellow, followed by the typical conspicuous black and blue tones. While most patients remember some form of trauma associated with their bruising, some patients do not recollect an exact moment of their injury
. In these latter cases, some form of trauma may have actually occurred without the patient knowing. This is often the case in older individuals with more fragile skin, or those who take blood-thinning medications like aspirin or coumadin. In other cases, unprovoked or frequent bruising can be a sign of some derangement of a patient's clotting system, such as abnormal clotting proteins or a low number of platelet cells.
I recommend that you seek out the help of your primary care physician
. Usually a reasonable explanation can be achieved with a thorough medical history and physical exam. In some cases, blood tests can be done to look at the clotting system and rule out any potentially dangerous underlying causes.