One of our members, @Karen, wrote up this explanation a while back of why your period may have come early or late during (usually) your first Whole30. I thought it was far too good to let it be lost in the depths of the forum so I've pinned it to the top of the Ladies Only section. Hopefully if you're a little nervous or worried about your early or "missing" period, this will help explain what may be happening. As with all things on this forum, no one here is medically trained and this information is not meant to be taken as medical advice. If you are nervous or concerned, please see your doctor. ~ Ladyshanny
Here's my synopsis of it all - someone else can chime in if I'm not 100% or if there's more to the story... I'm going off memory and not consulting my sources for the fine details. My cycles used to be all over the place and I took the time to figure it all out, but I'm going by memory here... This applies to those that AREN'T on hormonal birth control.
Although we tend to think of our cycles in terms of our actual period, ovulation actually runs the show. From the time you have your period until you ovulate, estrogen is dominant. Once your FSH and estrogen levels reach a peak level, your body decides, "hey, I can ovulate now". For most people, this takes about 14 days from the first day of our period to happen, but for some, it can take much longer. Things like stress (eh hem, diet changes!, stress from work/relationships, car accident, etc.) can actually prevent ovulation for a little bit while your body figures out what's going on. After all, it doesn't want to allow you to get pregnant while the body is under stress, so it holds onto that egg until it knows all is well.
However, once you ovulate, your body has a finite amount of time until you'll get your period. For most, that's 12-14 days. From ovulation until your period, progesterone is dominant. Your progesterone levels raise until it realizes your body isn't pregnant, and then when your progesterone levels drop, that prompts your period, and it starts all over again. Got it? (Interesting side note - progesterone actually causes your body temp to rise. That's why people TTC and trying NOT TC take their temps. When temps rise, you've ovulated, and when it drops, you can expect your period within a day or two.)
If you just started a Whole30 and your period is late, it's quite likely that ovulation was delayed due to stress. You'll still have about 12-14 days from ovulation until your period, so delayed ovulation means delayed period. Granted, there could be other reasons, but if you were completely regular your entire life and all of a sudden this threw things for a loop, that's a possible explanation. That's how it typically is for me. Another reason is that for those that are estrogen-dominant, you may not have as much progesterone, so instead of getting that 12-14 days between your period and ovulation, you might normally, for example, only 8 days. But when you change your diet and balance your hormones, poof, your progesterone levels kick in and you may get a few more good days between ovulation and your period! For those TTC, that's super important. But if you're just counting the days from your last period, it may seem a bit late.
For those of you that end up with your period earlier than expected while on a Whole30, there can be a few possible theories, but they all depend on where you are in your cycle and where your hormones are at. If your body has been trying to ovulate but it's taking longer than normal due to perceived stress, your endometrium is still thickening that whole time, and your body may need to release some of it (spotting). Your body could even say, "screw it, there's no way we're ovulating this month!" and your period could start without even ovulating (that's an annovulatory cycle). If you've already ovulated and your hormones are in flux, perhaps your progesterone levels have dropped temporarily, which means your period starts sooner than normal instead of getting those 12-14 days between ovulation and your period. I'm sure there are other reasons, but those are my best guesses for those that are curious.
Rest assured, at some point, your body will figure out that the diet changes are actually a good thing. Typically within a cycle or two, estrogen and progesterone will balance out how they should, your luteal phase (between ovulation and your period) will be the appropriate length, and all is well in the world. For those that are concerned about TTC, I HIGHLY recommend Taking Charge of Your Fertility by Toni Wexler. She explains all this, how to track your cycles (that's the only way I could make sense of my goofy cycles for a while), and how you can correct any oddballs that you run into!