I did not experience this when I became vegan, and I don't know of any other women who experienced this.
I calculate that you may be underweight. (I'm not used to lbs, so you might want to google for a "BMI calculator" and check yourself). A lot of people lose weight when they become vegan. Even if you don't feel very underweight, your body fat levels might be low.
The disappearance of menstrual periods is a common problem with extreme weight loss [http://www.bbc.co.uk/health/ask_the_doctor/periodcessation.shtml]
I think you should see a doctor: they will be able to give you a pregnancy test, assess your weight, and check there is nothing wrong.