Achilles Tondonitis may cas the tendon to thicken in certain places. If you are an athlete, this quite possibly could be the problem. Check with your doctor as soon as possible. This problem can easily be diagnosed with a physical exam. If this is diagnosed early, it can respond very well to treatment.
A bump or lump on the Achilles' tendon could be tendonitis or Achilles' tendinopathy. This is when the tendon becomes thickened in the mid-portion of the Achilles' tendon. See an orthopedic doctor and get some exercises to help with the problem.