1) We don't use SWIM here, we all know it's you and your girlfriend.
2) As tokezu said, doing MDMA 8 times in less than three weeks is extreme. I'd even go so far as to say more than extreme, straight up self-destructive. Do you/she not care about her mind and body? Fucking with something as fragile as your serotonin system like that is a perfect path towards brain damage. Seriously. Think a little bit, especially if you see yourself as hippies.
3) In terms of the actual headache I see three options. One, dehydration, especially possible and likely to be at least a factor considering all the drugs she's been doing and probably not hydrating enough on. Two, an actual issue with her serotonin system. Three, a psychosomatic reaction induced by either her body or her higher self as a last ditch effort to try and stop her from partaking in so many chemicals. We're smart like that.