I started my period when I was 9 years old. By the time I was 16 it still wasn't regular. I could go two weeks between periods, a month, three months, I've even had it go a year-and-a-half.
At 16 I was put on birth control and I had a regular period so long as I was on that. I'm now 24, nearly 25.
Recently, my fiance and I are trying to have a baby and when I went off the pill my periods came every two months; not a standard time, but it was like clock work. Now my last period was in January and it's now the middle of June.
My doctors think that I might have Polycystic Ovarian Syndrome; they've thought that most of my life.
I was put on Metformin (a drug for diabetes) that is supposed to help women with PCOS, but still no period. I've been on that pill since the beginning of April.
I know that it's not much help, but that's my side of a missing period. I'm not pregnant. I check every month, and boy are those tests expensive.
Good luck.