Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with a doctor.
Without the ability to examine and obtain a history, I can not tell you what the exact cause of the symptoms is. However I will try to provide you with some useful information.
Ultimately, the differential depends on the exact location of your symptoms. It sounds like you are describing the region of the trapezius muscle. This is a large muscle that extends from the back of your head down into the shoulder and further down into the back. Muscles can spasm, especially after a sudden motion like sneezing, causing pain.
There are concerning causes of pain in the neck after sneezing. One is a dissection, which is a small tear in the blood vessels that travel up the neck to the brain. This can occur spontaneously in people with certain conditions that affect the blood vessels, after neck trauma, or after chiropractic manipulation of the neck. The pain is often but not always associated with some sort of neurologic deficit as a dissection can often lead to a stroke. A dissection is diagnosed with a specific type of MRI test (MRA with fat saturation) or an CT angiogram. This type of pain is typically more localized to the neck and causes headaches.
These are just two possible causes. These and the many other causes would best be evaluated by your local physician. I would suggest that if you are still having symptoms, you should be evaluated by a physician more urgently.
Thank you for this opportunity to answer your questions, I hope you find the information I have provided useful, good luck.