I have an abnormal swelling on a scar. What's the problem?

 
That is probably a keloid. Keloids are formed when there is exuberant formation of scar tissue. We do not know why this happens. There may be a genetic basis for this. Keloids commonly occur on the chest, collar bone, back, and ear piercings. Keloids tend to occur on surgical scars, vaccination sites, ear piercings, acne scars, tattoo sites etc. There are multiple treatment modalities available for keloids, with varying efficacies. If it is bothering you, you should consult a doctor who can examine you and offer you treatment options.