The first time I wasn't really paying attention as I wasn't having sex so there was no real need to know but looking back it made sense cemented by the fact my Mum's cycle returned at a similar time (6months and there is 15months between me and my brother). The second time I was caught completely off guard when it showed up 6w whilst I was tandem feeding!!! I originally thought that no way this could be it but it has been regular ever since.