What is this painfull bump in the middle of my achilles tendon?

I randomly noticed it like 4 weeks ago at my track meet, being painfull when I decelerated. It would be painfull when walking in school. And then it seemed to subside throughout my brief "feel good" workouts during off season (right now). It started to hurt again today halfway through my approximently hour long run. There is a bump halfway on my left achilles tendon. The achilles itself appears swelled or enlarged, while my right one is defined and thiner. What should I do?