This is common in runners. The tendon which attaches the calf muscle to the heel becomes painful and inflammed.
The condition usually occurs due to stepping up a training programme too quickly. It can also occur due to poor foot biomechanics or due to poor footwear.
Increasing your training programe slowly rather than quickly will tend to prevent this condition. If you are a keen runner you should ideally change your footwear annually, if not every 18 months as the support in the trainer will show signs of wear. This could lead to less foot support which may increase the probability of developing Achilles tendonitis.
Physiotherapy treatment will involve soft tissus mobilisations, foot biomechanical assessment, advice on training and stretches and electrotherapy.