Tea is high in fluoride. Drinking lots of tea can be bad for you because the fuoride builds up in the body.
There was a report of a woman who was sick with lots of different symptoms but one I think I remember was joint pain. She went to numerous doctors who could not figure out what the cause was. Finally someone (maybe she did) made the connection to her high tea intake.
Fluoride has several mechanisms of toxicity. Ingested fluoride initially acts locally on the intestinal mucosa. It can form hydrofluoric acid in the stomach, which leads to GI irritation or corrosive effects. Following ingestion, the GI tract is the earliest and most commonly affected organ system.
http://www.emedicine.com/emerg/TOPIC181.HTM
The food either before or with tea is about the mucosa not be irritated as much.
Blood sugar problems do not make you nauseated.
You can get your doctor to check your blood sugar or you can look into fuoride's effect on the body and how to treat it.