What Causes Dark Skin With Burning Sensation On Foot?

hello doctor,i am a diabetic patient for past five years.now i have a problem in my upper surface of the left foot skin.the skin on my left foot gradually darkened with heavy burning sensation and spreaded all over the upper side of foot with pain.but it does not affect my fingers.it remains in good condition.now i have the pain around my foot joint also.please answer me what type of the problem it would be and suggest me a remedy.