What Causes Swelling And Bruising Of Foot In A Diabetic?

My 73 yr. Old mother has A swollen foot that is warm to the touch. She says it feels bruised to the point where she can't put her weight on it. She is also diabetic, with a podiatrist appt. Next Wed. We have been soaking in warm, then cold water and putting cold compresses on it for two days now. Don't know how severe it is.