The Achilles tendon is surrounded by a paratenon to help lessen friction with movement. You will notice swelling, thickening and tenderness in comparison with the other tendon, due to friction on the paratenon.
Your doctor may also be able to demonstrate crepitus, a crackling sensation in the tendon between finger and thumb as the ankle is moved.
Soft tissue x-ray should confirm what is already obvious, but may also show thickening in Kagars triangle, the area between the front of the tendon and the back of the tibia.
What else could it be?
It could be Achilles tendinitis, although it is important to remember that you may be suffering from several conditions at once.
Ice and gentle stretching should be the mainstay of your management after you have eliminated friction between tendon and shoe back.
This is one of only two conditions in which a steroid injection near the Achilles can produce dramatic relief. Provided your doctor inserts it into the paratenon and not the tendon, you can achieve a rapid resolution of symptoms and signs. However, if it is inserted into the tendon, there is a high risk of tendon rupture.
You should have a mandatory weeks rest after the injection, but if more conservative methods of treatment are used, training can be increased accordingly as the crepitus and swelling subside.