While doing bow pose, if you have experienced a headache or head tension, it may be because of intense backbend practice. To avoid, it is better to properly warm, stretched opening the arms, shoulders, legs and hips first before doing the bow posture. And you may open the spine in this manner – first the upper back followed opening of sacral and lastly the lumbar region. Alongwith, ensure that you should not hold breath unconsciously anytime during initial position, or final posture or while exiting from bow pose. Also just be aware about the correct alignment of the spine, not to squeeze any of the abdominal vital organs unevenly which can also be the cause of headache.
You can also go through our presentation on basics of asanas/pranayama as well as youtube video on how to avoid injury during yoga practice.
If the headache still persists, it is suggested that you should consult with any senior experienced yoga teacher or physician to discuss on this issue. Please feel free to write to us for further clarifications/information, if any.