Thanks for the information about the book, I'll see if I can find it!
I'm actually a neuroscientist, and one of my research areas of interest is how steroid sex hormones (like testosterone and estrogen) affect brain functioning. My particular interest has been hormones in the androgen family in men - there's far more research done about "estrogen brain" in women, but when people think of testosterone, they tend to think of hairy bodies and deep voices and sex drive -- not brain function. However, I believe that androgen is extremely important for proper brain function in men, and the lack of it is a bad thing. Thus, I would say I'm more of a testosterone guru than an estrogen guru!
I'm not an expert on hormones in terms of their effects on peripheral systems, and there are many, many more hormones other than the sex steroids or the steroid family of hormones. A change in diet can certainly affect your cycling, and I know in particular that the amount of fat in your diet seems to affect estrogen (as estrogens are necessarily dependent on fat for many of their metabolic properties). Thus, the amount of fat and the type of fat in the diet is the most likely to be affected by estrogens and to affect the cycle.
A low-fat diet tends to result in the excretion of more estrogen and estrogen metabolites from the system. A very low-fat diet can also cause irregularity in the cycle, especially if it's paired with a lot of fiber (basically, in doing this you're flushing out your estrogens). In contrast, a high-fat diet can cause more cramping and more protracted PMS symptoms in some women, but more regular periods (saturated fat, for some reason, seems to help the most with regular cycles. Go figure). Basically: Research has suggested that the more fat you consume, the higher your body's estrogen production will be. The less fat, the less estrogen. Fiber also plays a key role because it helps the body to flush out excess estrogen through the liver and bile duct: The liver pulls estrogen from the bloodstream, through the bile duct and into the intestinal tract, where fiber soaks it up and carries it out of the body with the rest of your waste.
You, of course, seem to have symptoms of both! Spotting could an irregular period (low estrogen) but the increased PMS could high estrogen. Thus, science is sometimes not an exact science. It may have to do with your balance of fats on particular days of the cycle, but I'm not a nutritionist nor a nurse or an MD (I'm a Ph.D.) and I wouldn't feel comfortable giving that sort of specific advice. Plus, there are plenty of other things that can affect your cycle (like stress, as you say).
Gaining or losing a lot of weight in a short time can also affect your menstrual cycle, but since you're only three weeks in and haven't lost a significant amount of weight, I doubt that your weight in general has much to do with it?
Estrogens do make it more difficult for women to lose body fat - the higher your estrogen, the harder it is to lose weight, especially around the belly and from "visceral" body fat (the body fat around your organs). However, it's natural for women to be higher in estrogen.
That's about as much as I know off the top of my head (and I double-checked the stuff about the liver, cause that was fuzzy). I'd say if it's bothering you, speak to your OBGYN about it.
Female, 30 years old, 5'4 1/2" tall
Starting weight 1/4/11 = 215.2 lbs.
Weight 8/30/13 = 179.8 (net: -35.4)
Current mini-goal: 175 lbs.
Lowest weight: 156.7 lbs.
150.2 lbs. <--- Official "Healthy BMI" weight
Estimated final goal: 130 lbs.
"You don't have to change your life today. You only need to change your day today."