Hi, Well, there are several possible causes. This may be a beginning of leg cellulitis which is inflammation of subcutaneous tissues and may develop into diabetic foot or even diabetic gangrene so any swelling, redness or pain in legs in someone who has diabetes should be evaluated by medical professionals since antibiotics may have to be started as soon as possible to prevent spreading of infection. He should do at least basic blood tests and blood sugar to see if there is significant infection or not. Other causes include deep vein thrombosis or lymph edema and these should be checked with an ultrasound exam. Hope I have answered your query. Let me know if I can assist you further. Take care Regards, Dr Iven Romic Rommstein, General Surgeon