Welcome to healthcare Magic!
The painful lump in the sole of foot is mostly due to infections due to bacteria or fungus, the inciting cause may be a forgotten thorn prick which later might have got infected.
If you are a diabetic then it may be a result of diabetic foot
Still the other possibility may be of plantar fasciitis
and local necrosis
leading to a continued inflammation and pain.
You should get yourself evaluated by a surgeon of your locality for an accurate diagnosis so that it can be treated accordingly.
I hope the advise would be informative and useful for you.