In reply to Hannah V:
Assuming it is an achilles problem there are a few things to do that I was shown by a physio :
1) Don't wear flat shoes (e.g..deck shoes) and consider putting insoles in other shoes.
2) There are a couple of simple stretching exercises that you can find on the internet to stretch out your calf.
http://www.runningwithhaynes.com/2012/10/12/stretching-routine-for-runners/
Do the first two (gastroc and soleus) stretches 3x30 seconds daily,
3) strengthen achilles by doing heel raises on the stairs (3x30 secs)
Don't overdo the stretches. If you can afford it go to a physio anyway in case there is something else going on..
My achilles is fine during the (downhill) ski season but plays up when I'm hillwalking and climbing and these things generally fix it.