First of all, well done for cracking the first week!! You've really done the hard part now!!
I can't really say what the headache may be. It can be down to many things; Stress, different sleep pattern, chemical change, dehydration, normal headache....even the fact you're taking on more oxygen, and less bad gases!
Not sure about everyone else, but I went through all kinds of odd changes; both physically and mentally.
You have to realise that what we did to ourselves over a long period of time, was not natural, and whether you thought it or not, the body was always working to rid you of it where it could....now it's sorting itself out, and even though it feels weird, you have to think of it as all part of the cleaning process!!
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