Most likely it is being caused by either irritation or a slight rash. Diapers that cover a belly button can and will irritate the skin of a newborn. Fold the diaper over itself. Also make sure her clothing is not too tight against her skin.
Sometimes the belly button can become irritated by clothing, soap, or water. Try drying it out with a little bit of rubbing alcohol on a Q-tip. If it continues to remain red, make sure to see her pediatrician.
It could be from having the cord cut and just a little bit of irritation from that and may be nothing to worry about. I would consult with a doctor if the redness moves or stays for several more days. But generally I'd think it was normal to have a slightly red belly button.
The belly button takes a long time to heal and look normal. I would recommend cleaning it good with alchol and putting vasoline on it. It will probably take a few more months to look normal.
