do some searches for past thredas as there is a ton of achilles horror stories on here.
basic thing is to be careful. achilles is a bitch of an injury to get over. heals very slowly and it is easy to make it worse doing things that you think may be helping. ignore it too long and surgery is possibly in your future.
the "miracle cure" for midpoint (vs insertional) achilles tendonitis is eccentric calf exercises (google or letsrun search will provide tons of info - some of it even useful!). basically stand on a step, lift up with both feet and come down with the bad foot only. 3x15 twice a day. add weights in a backpack as you get stronger.
ice, rest, stretch (but be careful or you just rip up all teh healing), heel lifts, nightsplints are all part of the fun.
of course you could have something completely different so the first thing would be to find a good sports doctor (preferably one who is also a runner) and have them examine it.
good luck