Hi, and thanks for your medical question on eHealth forum!
If you had normal periods then probably the headache and nausea is not due to pregnancy. However since some women get one or two normal periods and are yet pregnant, take a pregnancy test and see. Since you have neck pain, headache and nausea chances of a nerve compression in the cervical spine region is more likely. Get a doctor to check this. You may need a MRI and a neurology consult. Hope this helps. Take care!