My daughter is six weeks 2 days, she was born with a small discoloration on chest. which pediatrician said was ok and it will fade. now its worse on chest and on the chin. should i go back to pediatrician ? what could be causing this?

Hard to speculate: Infants are often born with birthmarks of various size & location. These can be harmless pigment changes or signal something more worrisome. Without seeing the mark or having a better description I cannot speculate on its nature. This site does not support our seeing pictures, so you will likely need to go back & have it re-evaluated if it bothers you.