What Causes Pain And Hardening Of Skin Under Foot?

HI Doc, i'm facing a problem in a small area under my left foot, (at the root joint of the extreme left finger), where the skin of 5 mm dia has gone hard and is paining. the pain goes even worse when the foot is on pressure specially when i have to stand for a prolonged time.I'm not able to recognise what exactly is it.
this problem had started to emerge some 3 months back. I'm not sure what is it and why is it happening so.Please Help!!Thanks Vaibhav