Do you consume any other caffeine normally? My understanding is that tea only has about half as much as coffee, but if you don’t drink coffee, soda, or another major source of caffeine that could do it and would be the most likely culprit I think.
It could also be an allergy or food sensitivity I suppose. Do other black teas do the same? Do you have a bad reaction to citrus at all?
Outside of those I’d look for things accompanying the tea that might be causing the problem. For instance if you normally eat cookies or biscotti with it, add honey or milk, or usually drink it after a meal, those may be a problem as well.