What Causes Pain On Bump On Achilles Tendon After Running, Climbing Stairs?

I just recently started baseball season and have started doing a lot of running. My Achilles tendon started bothering me, and is especially painful when walking up stairs. It hurts about an inch above my heel and there is something like a little ball on my tendon that's a little tender but not badly. My Achilles tendon feels a little stiff in the morning and only hurts to walk on after I've done a quick Sprint or after walking up stairs. It can hurt so bad that I have to limp. Other times it feels like nothing wrong and doesn't hurt like after I ran for a while or even after not even doing anything. I also feel creaking where the bumb is. Any idea what this is?