Please do make sure that you both use birth control and practice safe sex. A pregnancy can ruin or at least greatly affect both of your lives whether the girl decides to keep the child or terminate the pregnancy. Perhaps you cannot be criminally charged, but having a child is basically forever and, if you think that is romantic, think once more.
16 is the age of consent in NC. Therefore your girlfriend is considered by NC to have the ability to determine if she wants to have intercourse. Statutory rape does not apply in your situation.
Generally it's 13,14 and 15 years old plus six years older (or more) for the partner for statutory rape to apply in North Carolina. So even if you were 19 at some point when she was 15 it is not a crime in North Carolina (as you were less than six years older than a 13,14 or 15 year old). There are a lot of other things an angry Mom can do to make your life very difficult - I would advise against doing anything that would anger your girlfriends mother.