First let me just say I'm not a doctor or any type of trained professional with regard to nutrition or physical fitness...
I don't know what you mean by workout. This is with regard to weight training but some of this advice like breathing could carry over to cardio.
1) I would say hydration but you seem to be getting plenty of water.
2) Improper technique. The head and neck should be in a neutral position while exercising, unless of course you're specifically exercising your neck muscles. Your eyes should be looking forward and your head up, with your head inline with your spine. This means your head should not be tilted forward, back, or to the side. If your exercises are done incorrectly it could lead to tightness in the neck and tension headaches.
3) Over training can also lead to tension headaches.
4) Incorrect breathing. Be sure to exhale on the contraction phase and inhale on the negative phase. Do not hold your breathe. This can lead to headaches.
If this advice doesn't help you should probably see a medical professional.
The only way of finding the limits of the possible is by going beyond them into the impossible. -- Arthur C. Clarke