I have never been thin, but my cycles have always been irregular. My girlfriends always seemed to be able to look at a calendar and say, "Oh, not Tuesday, I'll be having my period," but I might not even have one that month. Even as an adult, I regularly went 2 or 3 months without a period. The closest I came to "normal" was having cycles every 40 to 45 days. Now, in my 40s and after my 4th child, I have cycles close to the "normal" 28 day ones, and I'm constantly thinking, "Again? Geez, I just had one!"
My college textbook said that once women have had 1 child, they never get cramps again. HA! I wanted to beat the (obviously male) author with a copy of the book. My periods are always painful. Maybe it's related to the irregularity - I don't know. Luckily, it's usually just a day or so, not the whole period.