S.B.
My now 13 yr old daughter got her first period last fall. Since then, she has only had 3 cycles. My older girls were the same way for the first 6 months to a year. I started menstruating when I was 11 and then it stopped for a year when I was 12. I was always told all of this was normal and that some girls aren't "regular" for a few years (or longer, in some cases).
I did ask my 13 yr old to write down when she does get it on her calendar and for how long just in case we need to know for some reason (like at a physical or something).
Good luck!