It is a tricky one. Starches, and potatoes in particular cause blood glucose to spice even more so than table sugar. Potatoes are a high Glycemic Index (GI) food. http://www.glycemicindex.com/
A baked potato will cause your blood Sugar to spike 4X more than eating an apple.
So between the gut fermentation, and the amount of insulin that your pancreas has to secrete to counter the glucose spikes, no wonder you feel lightheaded.