Some intense sensations can emerge during mindfulness practise for numerous reasons, including posture and old injury. They can be distressing at times, particularly when you are required to keep still and focus on body sensations during body-scanning techniques.
The best way to face possible pain when practising mindfulness is to direct your attention to the centre of the intense body sensation. At first, this can enhance your perception of the pain, but it will decrease as you apply equanimity to the experience, partly because you will stop interpreting unpleasant sensations as threats. This will help to reduce stress hormones in your body. Try to resist the urge to do things to avoid or escape sensations of pain. Reacting with avoidance will only make it appear as though the pain is strengthened or unchangeable.
It is always good to remember that, like thoughts and all other experiences, body sensations are also impermanent by nature. No matter how intense or painful they are, they eventually pass away.