Dizziness hours after erging

Hey guys I was hoping you could help me out here. I started erging again, after 6 months of inactivity, and I'm following concept2's 2000m program. I've been at it for one and a half month, and up to now (4 hours/week, 3 workouts at 80% of vo2max and one easy, approximately 2500 kcal/week according to my suunto hrm) I was feeling fine, but hours after my last 3 workouts, I've started feeling lightheaded and at times dizzy.

I looked around and the most common cause was dehydration. So, I then changed my hydration habits during my last workout (lots of fluids, before and after exercise, sport drinks etc) but 8 hours later I got this lightheadedness again. Could it be nutrition related, or is it something obvious I am missing? Should I give up the 2000m program and follow a less demanding one?

E D I T : Talked to my doctor. He said I'm overtraining, and that the dizziness comes from stress on the head muscles. He said to take it easy. Just adding this for posterity.