Let me add that the problem arises only after eating sugar and nothing else. She can eat large amounts of bread, cheese with none of the symptoms.
Blood sugar problems are not confined to diabetes. Alcohol is mostly sugar. Since sugar isn't good for you anyway and it doesn't agree with her, why eat it? I mean, once in a while for a treat, but it isn't working for her, which is great since there's not benefit to it. But again, it sounds like a blood sugar problem one way or the other.
Obviously avoiding sugar altogether would circumvent the problem, but we would still prefer to know whether or not she has a medical condition.
Yesterday she measured her blood sugar which was 125 mg/dl [6.95 mmol/l) two hours after eating and 95 mg/dl [5.27 mmol/l] another two hours later. If I am not mistaken, these are healthy levels.
There are a lot of things that happen to a body that are hard to discover the cause of. You can look forever and not find it. But you can look -- but you'll need to see someone with expertise, which is not usually your regular doc. It just depends on how far you want to pursue something or just avoid the cause and also avoid what could be a long expensive search. Up to you. Could be a lot of things. Could be food intolerance, something other than just the sugar. But if she avoids sugar and feels better, you've solved the problem and made her much healthier to boot without undergoing a lot of testing. But again, if you're concerned, the only way to do this is to see specialists.