Osteopath
There are lots of reasons for headaches & nausea, stress is one & it is a far more common cause than arnold-chiari, do you have a reason to believe you have that? If you do then headaches are usually very very severe, other symptoms include loss of sensation & loss of voice, or changed voice amongst other things see http://www.ionosteo.com/ionblog/bid/206247/Syringomyelia-Arnold-Chiari-Malformation for a case study (at end of that link)
MRI would confirm one way or another, but the doctor won't refer you unless he can see symproms to warrant it. I would reccomend you see a local osteopath or chiropractor & get a physical examination often neck problems can cause headaches & nausea, You might have what is called a cervicogenc headache which can also cause nausea, here's a link on that
http://www.ionosteo.com/ionblog/bid/112964/Cervicogenic-Headaches It is a far far more common condition.
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