Do I need to add formula?

Lydia
My LO is 13 days old, and feels like she's growing fine! She nurses for about 15-20 minutes and gets a nice full belly! Afterwards I can pump about 8oz (from combined breasts) sometimes more sometimes less. Anyway! The last 2 days she's been cluster feeding a little, sleeping A LOT (even for a newborn I feel) and has been a little fussy. Is there any reason I should suspect something wrong with my milk? Do I need to supplement?