That depends on the person. I've heard a lot of people just don't tolerate a super high carb jolt. I personally don't throw up, but I do feel like total tired trash the next day. Sometimes my stomach gets a little upset, but I've as yet not had a carb cheat not followed by whiskey, so I can't be sure it isn't the combination of the carbs and alcohol and not just the carbs.

Whiskey with low-carb food doesn't cause that for me.

I'm so sorry you felt so sick.

I don't think it's "now intolerant" so much as it's "no longer putting up with it" and "can no longer be ignored."

Most of us have had the "poisons" or other not-really-food for so long that the body has long since given up trying to say "hey, wait, this isn't kosher," and either puts up with it as best it can, or plagues you with chronic issues that are just dismissed as "bad genes" or "just getting older" -- the eczema, the RA, the cluster headaches, etc. You're so used to the way things are, and you consume those triggers so often, that you can no longer associate them with any given food.

But then you changed the game when you went LCHF. Suddenly, you're no longer eating those triggers, and your body feels what it's like to not be in a state of chronic pain or inflammation, and since it's not in that chronic state anymore, it doesn't have to continue to suppress those red flags you ignored for so long. Then, when you do eat those trigger foods, your body throws those red flags and you get the full force of them (instead of the dampened one), and the difference between "no inflammation" and "this food triggered this reaction" is a lot more stark than that between "constant, low level inflammation from chronic intake of trigger foods" and "this food triggered this reaction" (ie - going from 0 to 7 is a lot more of a stark difference than going from 5 to 8, even though 8 is a higher level).

Another way to look at it is the tolerance level of a drug addict -- the body always reacts with "this is bad!" when you give it something like heroin. However, a long-time user has built a tolerance, so something like 5g (pulling a number out of the air, here) just gives them a nice high. Put that person in rehab, though, and get them clean for, say, six months. Then, they slip up and go back to the drugs. This time, they take their old usual of 5g and it kills them of overdose. Did they suddenly become intolerant of the effects? Not in the medical sense of "intolerance." Rather, they lost the tolerance they developed from their chronic exposure and having gradually increased the amount they took.

The body is very much a "use it or lose it" machine. When you build a tolerance to something, you build mechanisms to handle it in one form or another (or at least to cope with it). When you stop consuming that thing, you no longer need those mechanisms, so the body breaks them down. You can build them back up again, but it takes either a hard switch (like you might have done when going LCHF) or gradually increasing the amount (like people do when re-introducing a food, or like the heroin addict does at first in the example above).

Maybe its a reverse effect. You know how people get the "low carb flu" when they first start out, maybe its the opposite.

Yes! It's never made me throw up, but it does, erm, run right through me...
If I take in more carbs than 60 or so, I can't run the next day unless there are plenty of restrooms on the route.