Hello there we understand your feeling and your concern towards you mother. This condition is quiet common in peoole with diabetes but you see one cannot take it lightly. You need to get her evaluated. But then see there are two scenarios and you need to take her to two different doctor in bith if them. If the foot is red and the skin is thick and if she unable to walk then you need to take her to a general surgeon. If the swelling is on both the legs and then she is having breathing difficulty or difficulty in voiding urine then you need to take her to a general physician.