Hi Anna,
That's interesting, as I've had the same problem and the docs can't find out why. My hormone levels are all normal (been checked twice) and I've had an internal scan which was fine.
I had the problem for just over a year. I would have 3 normal cycles then have one where I was always 2 months late and would have a horrendous, nagging toothache like pain in my ovaries until I finally got my period. The problem started when I started to take my running a bit more seriously about 18 months ago and started running more regulalry and built up to a half marathon.
Sod's law......as soon as the doctor booked me in for a scan at the hospital things went back to normal of their own acccord. They never did get to the bottom of it......but, touch wood, I haven't missed any since last October, although I do occasionally get the toothachey pain.
Sorry that doesn't tell you what it is, but at least you know it's not just you
.
Might be worth you going to the docs to get checked out though, if only for piece of mind.
Saffy xx