Achilles tendonitis is, as you probably know, a type of overuse injury to the achilles tendon. It is one of the overuse injuries that is hardest to heal up completely, mostly because the achilles tendon is so large and has so few blood vessels, meaning that there simply aren't enough oxygen and nutrients delivered to it to allow it to heal quickly. For this same reason, it is very important not to "run through the pain" because tendonitis that is continually strained can increase your risk of an achilles tendon rupture, which would be a very serious medical complication, often requiring surgery
The main cause of achilles tendonitis is just too much exercise (running too much on hard surface for example). In rare cases a leg length discrepancy might be the cause (your doctor
should be able to figure this out for you). Another major cause is over pronation, which is when your foot turns inward when you run or walk. An athletic shoe specialist or athletic trainer should be able to diagnose over pronation by examining your gait.
I would start by going to see your primary care doctor
. They will be able to help you figure out what the next steps should be.