Sounds very much like tendonitis. Inflamation of the tissue sheath around the achilles tendon.
Associated with dehydration on long rides, drying the lubricant around tendon - and inflaming with continued use. Most prevalent in older blokes doing endurance audax type rides in warmer weather. Often involved with tight calves and suchlike.
If you carry on using it in this state - risk of a tear in the stickinig sheath, and adhesion - cue bigger problems. I know from experience.
Rest, hydrate, cool. Try glucosamine additives, or mussel extract. Use diclofenic but not in order to get back on bike - save that till its completely clear.
When you start riding again - increase pre ride loading of isotonics.