Gain around the middle means...
Considering your changes in metabolism, potential adrenal exhaustion and the places you are gaining the most weight, I suggest you incorporate sea veggies.
Often weight gain in the stomach area, when previously you had not experienced any gain during normal times, indicates that your thyroid needs some nutrients.
Give that a try, you can add Kelp powder to your recipes, eat dulse-add it to salads, or take encapsulated kelp or wakame.
Menstrual cycles in humans vs. other mammals
Other mammals are not a valid comparison to humans. Only humans, great apes, bats, and shrews experience true menstruation. All the others get estrus. That's why they don't have periods like us. Not because they are "more natural" than us. I'll refer you to the relevant wikipedia article.
From my understanding of human female fertility, in general, if you are not menstruating regularly it is because the body does not have the energetic resources, or maybe your hormone cycle is not "normal" (yes, I know that's a very subjective word!). If you're going 180+ days between cycles, you're experiencing amenorhea. This can happen when you're breastfeeding--it's hard for your body to nurture and grow two babies at once with success. Female athletes can experience it too, maybe because their body fat gets so low, or they use so much of their energy in activity.
Your menstrual cycle is actually a really healthy thing, not an inconvenience. I think that in this age of the pill and feminine hygiene and all that pap we are fed on TV, we are becoming less and less aware of the power and sacred nature of our feminine bodies.
If you have really painful cycles, your hormones could be unbalanced. A good naturopath could probably help you with that. If I was a fruitarian and I stopped having a period all together, I would definitely be concerned.
We are not told this in sex ed, but our reproductive hormones are actually really powerful aspects of our overall health. For instance, did y'all realize that having more kids, younger, and breastfeeding them longer can significantly reduce your chance of getting breast cancer? And that amenorrhea is associated with low bone density?
I just think that sometimes we get all caught up in spurious arguments about the benefits of this diet choice. But because we are making this choice, we have the responsibility to research things (at least check out wikipedia on stuff!! It's free and easy and a good start!). Just because someone or some website happens to concur with our choice of diet does not mean that they are infallible or that we don't need that grain of salt.