See a podiatrist!! And when you find one, ask first if they treat diabetic foot problems! Not all of them do and you could end up wasting your time.

Could be any number of causes- rheumatoid arthritis,neuropathy,plantar fascitis,tendon injury,ill fitting footware,trodden on something........see your doc.

Absolutely good advice. See your doctor. Get to the source of the problem, so it can be treated properly.