Sole Runner, I feel for you. But I would not stretch it right now! That very severe tightness is a warning sign and I'd say stop running straight away. I had an achilles tendon problem that started as a dull ache and then got much worse during a single run, to the extent that I could not walk without limping and had 7 months off running.
I'd go with ice, as it's inflamed, and ice will help. You could stick your whole foot in a bucket of water with ice to make it as cold as you can bear. You don't have to have swelling to have a very serious achilles problem. Sounds like you're on the cusp, and what happens next is down to the decisions you make.
So, what'll it be - a complete week off running to see how it feels after a rest, or plough on and risk severe damage?
There is a place for stretching in rehabilitation (see eccentric calf stretches) but only once soreness and tenderness are below a certain threshold...
Good luck and let us know how you get on.
Edited: 30/09/2012 at 13:18