Damage to the achilles tendon is pretty common in heavy runners. However, this generally manifests as pain within the tendon, and it should not cause redness of the overlying skin.
Therefore, I think a problem with the skin itself is more likely. One possibility would be irritated skin and callus formation from heavy running and the friction that your shoes are producing on the backs of your ankles. If it seems like the bump is right where your shoes rub and if the skin is thickened there, then this is the most likely possibility. Solutions would include putting moleskin over the area or getting a pair of shoes that don't rub the back of your ankles.
A second possibility would be that you have developed a skin infection. If the area of redness is spreading, or if it is warm to the touch or if there is any pus, then this is likely. If you suspect a skin infection, you should go see your doctor
, because skin infections can sometimes require drainage of pus collections or taking oral antibiotics in order to get the infection under control.
I would start by getting in to see your primary care doctor