I get extremely irritable when hungry. I also get extremely tired after having sugar--for instance, 30 minutes after drinking a coke I will have to take a nap. My doctor has done blood tests and says all is normal. What is wrong with me?

Glucose fluctuating. Irritability can be a symptom of low blood sugar. And if you consume a high-sugar food then your body responds by releasing insulin, sometimes in large amounts, to deal with the sudden spike in blood sugar. This can result in a drop in blood glucose, the so-called "crash" from a sugar-high. All of this can be a precursor to Insulin resistance and/or diabetes. Eat more fiber/whole grains.
Endocrinologist. If your physician does not know the cause of the problem, you might want to contact an endocrinologist.