I'm going to also guess it's Dehydration. I've been taking dietetics for about 9 years (HCTZ just changed out for Spiro!), and I've found if I don't stay up on my hydration, I feel Weak, Lightheaded, Hungry, Agitated, In Pain, and just Blah. So pound down that water.
And Water is best. Caffeine also dehydrates.