how long has it been knocking around?
Treatment is rest and ice initially, but you'll want to start eccentrically loading it fairly soon. Can also try bowstringing which can be really nice in reducing discomfort in the area.
Eccentric loading – stand on a step with your heels hanging off the edge. Push up onto tip toes using only your non painful leg, then slowly lower both of your heels until they are hanging off the step and you'll feel a nice stretch in your calves. Do about 10 reps (but let pain be your guide)When this becomes pain free progress to pushing up with good leg and coming down on the injured leg.
Bowstringing – bend your ankle up (dorsiflex) so that you can feel a stretch on the achilles. Place one thumb over the other on the medial (inside) edge of the achilles tendon itself, above the insertion onto the heel. Slowly and rhythmically with a constant pressure move the tendon towards the outside of the ankle (i.e. move it in the lateral direction).
Do this for ~ 30 sec at frequency of 1 / sec.
Also will want to work on some soleus and gastroc stretches
Hope that helps, and hope it gets better soon