I was going to suggest dehydration too...nursing takes sooo much fluid to maintain your hydration and produce milk, especially if you are tandemming....I know with ds one I was lucky, I never had any letdown reflex that I noticed except that the second i would start nursing him I would feel like if I didn't have a litre of water RIGHT NOW!!!! I would keel over dead. DH never could understand why it took me at least 6 months to remember to get my own water before sitting down to nurse...( I say I was just trying to include him in ds's care :LOL )
Also, if you have started drinking caffeine since baby was born, and weren't drinking it during pregnancy, or if you have changed your caffeine consumption in any way...that can contribute to intense headaches too....did you know that going from half a cup of coffee a day to nothing can cause intense caffeine withdrawal headaches?
Hope you find a solution soon.....I know how agonizing head pain can be.