Why would diet mountain dew make my blood sugar high?

It doesn't.: I would look for other reasons why your blood sugar is high, such as 'hidden' carbohydrates in fruits, juices, and starches.

It doesn't.: Diet mountain dew does not contain many calories at all, and no carbohydrates. Therefore, it cannot raise your blood sugar. I would look into other foods you may have eaten around the same time for a culprit as to why your sugar levels may be rising.

Insulin Resistance: You probably have some baseline insulin resistance. Perhaps it was some other factor of your meal or overall lifestyle and exercise level. You did not mention a diagnosis of diabetes, or what your weight is but it sounds like you are watching your diet and nutrition. I am glad you are engaging in your health and accepting responsibility for it!