Drinking baking soda is a bad idea for so many reasons. One you are going to neutralize your stomach acid leading to a number of health issues:
Secondly that is a lot of sodium, which can lead to an electrolyte imbalance. It is the excess sodium that is likely causing the headache.
And drinking baking soda can lead to a state of alkalosis, which is also very dangerous.
This is why I have such a problem with the Acid/Alkaline forums. These forums are not about balance but rather about creating an alkaline state, which is not as healthy as they claim and as you have seen can be quite dangerous.
Lay off the baking soda and allow your system to recover.