Brief Answer:
Neck issues most probable culprit.

Detailed Answer:
I read your question carefully and I am sorry about the symptoms your are experiencing.

If you have severe neck issues it is common to have symptoms similar to yours like headache, nausea and balance issues. That pain happens due to compression of spinal nerve roots leading to pain in the neck and head. The balance issues (dizziness and nausea) can be either due to compression on cervical vessels supplying the posterior part of the brain with blood or due to compression of nerve roots which bring signals to the vestibular nuclei (brain stem nuclei which are important for balance). Nystagmus is a possible accompanying sign.

So in my opinion the most probable origin is your cervical spine for which treatment is with anti inflammatory drugs and physical therapy.

Less likely is a stroke in your posterior brain, but considering your neck issues, your young age not typical for stroke and not mentioning other risk factors like high blood pressure, diabetes, heart arrhythmias, smoking, high cholesterol, recent head or neck trauma etc, that is not likely. Also usually there are other accompanying symptoms and signs.
If the new symptoms persist though, a physical evaluation should be performed by your doctorto check for neurological signs and to evaluate the need for brain imaging to exclude brain lesions

I remain at your disposal for further questions.

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Brief Answer:
Read below.

Detailed Answer:
It is good that you are checking with your doctor just to make sure. I understand that nystagmus can be scary symptom, but it can be present in severe cases as I said.
It can also be present in stroke but usually with other symptoms as I told you. Another common potential cause can be also inner ear issues but with the headache and history of neck problems I considered it not likely and didn't mention.

Wishing you good health!

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