In general, my observations are that women on hormonal BC find their period comes early when they make diet changes. Women that aren't on hormonal birth control often experience late periods. Here's a post where I went into detail about why diet changes mess up hormones, both for those on BC and those that aren't: http://forum.whole9life.com/topic/6028-i-just-started-my-period-8-days-early/ I'm very passionate and a little long-winded when it comes to hormones, so my apologies.
Freelie - I'd suspect your doctor won't have much to say. Although doctors often recommend diet changes to women with PCOS to help balance hormones and regulate cycles, they typically don't apply that same logic to other populations of women. As a result, they often don't have an explanation for period changes due to a clean diet... Most doctors that I've talked to are clueless and think hormones and diet are mutually exclusive. *smacks forehead.
To be clear, diet changes like this are not bad for your menstrual cycle. They really aren't. The changes may cause different things to happen in your body than they have before, but isn't that the point of eating clean? To get your body back into balance? I mean, there's loads of info in ISWF about hormones because of this! Think of this early period as a sign that things are working out the way they should. Getting your period early is a hiccup along the way and it won't be permanent!