In reply to purplemonkeyelephant, Adam_42:
I agree with the above - ask a doctor. However, if it is some kind of insertional achilles tendonitis/opathy, you can expect the key treatment to be heel raises and calf stretching. That treatment worked for me.
The heel raises both strengthen the calf, and (the theory is) promote the regeneration of tendon fibres with the correct orientation. The eccentric (lowering) part of the heel raises is what does this I think... I believe some dorsiflexion (i.e. do it off a step, so your heel goes below your toes) is key.
Shoes with a higher drop might help reduce the stress on it. Check your pronation and whether you need arch supports, as overpronation can put the achilles under increased stress.
It requires patience, as there's not much blood flow round there so it's slow to heal. For me to get back to running took several months of calf stretching plus 3 sets of 15 heel raises on each leg, once with straight leg and once bent knee, twice a day.
edit:
Response to Timmd - If it is damage to the tendon, physios thumbs might not help. I was told poking it/physio massage would just flare it up.
Post edited at 17:46