You need to see a doctor (preferably an oncologist). Worst case scenario is cancer and in that case the sooner you get treatment the greater chances of long-term survival. I strongly recommend you get the "lump" cut out and tested.
I am a 13 year survivor of a similar "tumor" that was in my leg (my cancer was liposarcoma). Note that tumors do not have any "feelings", but can cause pain as they grow and push against surrounding areas.
This could be a number of things. I would recommend consulting a physician about this.