It is doubtful that this change has anything specifically to do with breastfeeding. Keep drinking lots of water. Make it a habit to drink at least 8 to 10 glasses of water each day. Many of us just don't drink enough. Even slight dehydration can leave you feeling fatigued, and may very well affect your complexion.
Regular exercise is important.
Moderate aerobic exercise, such as walking, will increase your blood flow, resulting in a glowing complexion. It's great that you are already eating well. Skimping on nutrients may not be affecting the quality of your milk, but it can take a toll on a new mother. Be careful out in the sun. Use a sunscreen or a makeup with a SPF of at least 15.