Yes, it's pretty clear that some horses are more sensitive than others, on subjects of colic, insulin resistance, laminitis, etc. 'New pasture' can be problematic because it may be rich & high in sugar, causing hind gut acidosis, but even as in your case, if the pasture is overgrazed, or grass is otherwise stressed, it can be problematic because stressed grass can be much higher in sugar. So only allowing her restricted grazing is a good move IMO, even when it's overgrazed.
Re eating of pokeweed because the horse was in a 'diet yard', not saying this is the case for yours Hagon, just some info for those who don't know... Regardless of whether the horse needs to lose weight or not, it's still vital that they get free choice or little & often forage(horses still need at least 1.5%bwt daily). It could have been periods of hunger, not some weed(I don't know what pokeweed is) that caused the tummy probs. Feeding hay that's been soaked & drained to leach out a lot of the sugars, &/or using a 'slow feeder' such as a small holed hay net, to allow the horse constant access to feed but he needs to work for it, can't gorge, are options to 'diet' a horse without them going hungry.
Loosie, pokeweed is a weed common in the Southern part of the US. The early spring leaves are often used in a salad (Poke salad) that folk medicine says is sort of a "tonic." It can produce a mild bellyache that results in diaharrhea and it will clean you out. Spring cleaning for your body? Personally, you couldnt' PAY me to eat it and most horses will avoid it like the plague. That's why you see it growing up around fence posts, etc. It doesn't get eaten! Except for my pig. He has hay available to him (We go through this every Spring when the Spring grass comes in), he just wants GREEN! I battle his weight every Spring when the grass comes in. In the Fall, he adores all the acorns that fall from the oak trees in his pasture. It's not only pigs that will fatten on acorns --both of my horses love them! We rake and rake and rake... Oak trees are everywhere around here and so far we've had no problems...except for my horse's weight. I can tell you from personal experience that acorns are not poisonous.
As soon as we figured out what was going on, we declared war on the pokeweed and eliminated it. Since then, there have been no more bellyaches or colic. Big likes to chew. I'm just grateful he isn't a cribber and leaves the wooden fences alone.
I have an OTTB that's a really hard keeper that we've had for 11 years, and then my horse that is an air fern. Life would be MUCH simpler if they were both the same...one or the other, easy or hard, I wouldn't care. It would be even nicer if I just plain had normal horses!!!