Since they haven't ever been regular before and you feel well, does sound like just a "normal" glitch. Your weight doesn't sound too slight; I had regular periods at your age, was 5'6" and 127 lbs. and had no fat whatsoever (couldn't do "dead man's" float, literally sank to the bottom of the pool :-)

I don't know if you do a lot of exercise or anything; my girlfriend was a physical ed major and always running or doing sports and would miss hers because of all the "extra" exercise the doctor said; runners often miss theirs I think.