Did you have any swelling before the surgery - a small lump perhaps?
Can you feel any lump now or is the whole toe swollen and painful?
Do you have pain all the time or only when you press on the lump?
The commonest cause of a pain on the ball of the foot under a toe is the formation of bone spurs.
Bone spurs are extra bits of bone that form near joints, usually joints that are affected by arthritis. Constant friction, as by the active life you have led, can also cause minor trauma to the bones of the foot leading to spur formation as a defensive mechanism.
They do not, by themselves cause any problems but causes pain when they press on nearby ligaments or tendons. Since you already have arthritis of the knee, it is possible that you also have arthritis of other joints, like the ankle and those of the toe.
An Xray or a CT scan should be able to diagnose the bone spur quite well.
If the spur is causing a lot of problem, it will need to be removed.
To know more about bone spurs, please see:
I hope this helps.
Let me know if you need more input. If not, please click on the green"Accept" button on this page. A positive rating and adding a bonus will be warmly appreciated.