Open Question: What causes chronic neck pain?

I am a 24 years old, good health (minus-year-old female GERD) and I suffer from chronic neck pain. I have a good diet, exercise, 5 times a week, do yoga, and no bad posture. I have never hurt or accident. I've seen in the past 7 months, were only the first few days after adjustment chiropractor relief. Then the pain came back. I have never been diagnosed with osteoporosis, arthritis, fibromyalgia, etc. What could cause this chronic neck pain, I should of these conditions / diseases worried? Thank you!