I have ankylosing spondylitis and have experienced a tender vertebra for the last 6 months. could this be progressive as? other as symptoms are mild.

Possibly: With A/S your vertebral joints cannot heal as quickly when strained, hence, become inflamed and tender more easily. Speak with your rheumatologist about reducing the inflammation with alternative (turmeric) or prescription agents NSAIDS. Pain docs can also inject specific joints to ease your burden. TNS units, topical anti inflammatory (flector patch) agents can help too.

Potentially : There are several reasons a person can have increased pain if they have AS, including a "flare up" of the AS, a vertebral fracture, or the development of a condition known as spinal stenosis being among the most common possibilities. Don't hesitate to be seen and potentially getting an MRI to see the likely cause in your case.