Pressure on neck causing nausea

For the past year or so anytime I wear something that puts pressure on the back of my neck it causes me to feel nauseas & something weak, dizzy & headaches.  I have to refrain from wearing necklaces, shirts with collars & even a lot of jackets/coats/hoodies that would rest on the back of my neck.  I have went to have it checked out & was diagnosed with TMJ.  I was given muscle relaxers, mouth pieces to wear day & night to adjust my bit to relieve tension on my jaw muscles & joints since I clinched my teeth when stressed.  Which it did help, I no longer get the migraines I was from clinching my teeth & no more popping jaw all the time, but I am still having the neck problems... stiff neck & any pressure makes me sick to my stomach.

[Caryopteris]

It sounds to me like you have been clenching more muscles than just your jaw.  I suspect you would benefit from seeing a physical therapist for evaluation and treatments such as massage therapy.  You may also need assistance with learning relaxation techniques, such as using biofeedback.