What Causes Recurrence Of Bruising Around Lips?

i have recurrence of what appears to be bruising around the lips. This has been happening for quite some time now: approx. 3 years. It is unsightly but never painful. The skin is not broken and the dark bruises go away in a couple of days. There is no swelling or discomfort of any kind related to the occurrences and I have been unable to figure out what diet, or circumstances might be causal.