There could be many diagnosis for this lump like a cyst, lipoma, calcification etc. But since it is freely mobile and had been there for a long time , it seems like benign condition. If it was some cancer or tumor it would not have remain stable for so long.
Since it is causing discomfort recently, better to see a doctor and confirm the diagnosis. If you keep on touching the lump repeatedly it will be inflamed and cause pain due to irritation. So avoid repeated touching.
If the pain is due to some infection in the swelling, then a course of antibiotic will take care of it.
I hope this helps you.