The frictional force of the cannula against the skin in liposuction can create post inflammatory hyperpigmenation. Usually this fades but can take many months to even a year or more. Often the resolution is hastened by the application of topical lightening creams, such as Elure, hydroquinone, tretinoin, etc. See your doctor.
Web reference: http://www.thenyac.com/liposuction/index.html