Why does too much candy cause nausea?
There can be many reasons about why too much candy (chocolate) can cause nausea. Lets talk about them one by one.


Hyperglycemia - chocolate contains sugar, and too much sugar consumption can cause high blood sugar levels i.e. hyperglycemia. When blood sugar levels remain high for some time, it can cause nausea and vomiting, among many other symptoms including dizziness, itchy skin, dry mouth, thirst, etc. If left untreated, it can lead to diabetic ketoacidosis or diabetic coma. See this article for more information.
Artificial Sweeteners - often chocolates contain artificial sweeteners instead of table sugar or glucose to prevent risk of hyperglycemia without compromising with taste. But many times, these artificial sweeteners can cause side-effects too. For example, there have been reports that sucralose causes nausea and vomiting in some people. Other alernatives, like mannitol, sorbitol, xylitol, maltitol, isomalt, etc. can also cause nausea and similar symptoms.
Dumping Syndrome - sweet foods, like chocolate, are hyperosmolar i.e. have high osmolarity. When these are ingested in large amounts, they cause the small intestine to expand quickly, due to which the ingested food passes too quickly through the stomach, largely in undigested form. This condition is called Rapid Gastric Emptying (RGE) or Dumping Syndrome. The initial symptoms of dumping syndrome include nausea, dizziness, diarrhea, fatigue, etc. See the Wikipedia page for more information.
Fats - chocolate contains cocoa butter, and cocoa butter is a source of saturated fats. Ingestion of large amounts of fats can lead to indigestion, and indigestion can become a cause of nausea and vomiting. This also explains why greasy foods can cause nausea; because they have high fat content.


Some other factors could include candy allergy, pregnancy, etc. or something I failed to mention. But I hope this much would help too.