sounds like your achilles tendon is stretching. mine snapped and detatched. be careful and see your gp
Your Achilles' tendon is probably too tight from wearing heels or sitting too much or exercising without stretching.
Do not ignore this problem. One doctor you could see is a podiatrist. You could see a personal trainer or a physical therapist. It is a very common but serious problem. If you ignore it, it can cause heel spurs. Be very gentle with your movements to not aggravate it, but you will need to do some gentle stretches.
Good article:
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002067/