Causes of amenorrhea?

I my period is late. I know I'm not pregnant. At my last Doctors appt. the nurse said I was a little underweight. 112 lbs and 5'6'', meaning my bmi is 18.1. Is that enough to cause amennorhea? I'm not exercising heavily, or at all really. I'm only 20 years old, so it's not menopause or anything. Do you think it's my weight or something else?