What's actually causing it
Your Achilles is the thick cord joining your calf muscles to your heel bone, and every step and push-off runs through it. Tendinopathy is a painful overuse syndrome of that tendon 5: the load going through it has outpaced what it had adapted to carry.
That overload rarely arrives on its own. Things that make it more likely include an increase in training or activity, unsupportive footwear and high heels, flat or hyperpronated feet, carrying extra weight, inflammatory arthritis such as ankylosing spondylitis or psoriatic arthritis, and quinolone antibiotics 5.
How far it has progressed matters too: mild cases hurt during or shortly after one particular activity, moderate cases bring swelling and sometimes a hard nodule you can feel in the tendon, and in severe cases any weight-bearing activity hurts 5.
The other thing worth knowing is where your pain sits: insertional within the first 2 cm of the heel-bone attachment, midportion more than 2 cm above it 1. That isn't pedantry. Almost all the good treatment research was done on the midportion kind: in the largest comparative review, 86% of trials were midportion and no combined analysis could be run for the insertional variant at all 2. If your pain sits right down at the heel, the evidence behind any plan is thinner.