I asked this morning down at the market about the Diet Mountain Dew problem with their fountain. It turns out that the CO2 had run out so I had nearly straight syrup! It appears that the excessive sweetness triggered a cephalic insulin response (the bit of insulin that a body releases in anticipation of incoming food) and dropped my blood sugar down. Low blood sugar is what it felt like.
It's odd that I don't get the same response with regular Diet Pepsi. I have been drinking it so long that maybe my body is accustomed to that level of sweetness and "knows" that food isn't coming with it. Still, it's a good idea to cut it out totally to keep the insulin as low as possible.