My Mom, 90 Yrs Old, A Diabedic For Over 40

My Mom, 90 yrs old, a diabedic for over 40 yrs, 5'4 230 lbs has problems with her ankles swelling. Recently I noticed her skin bubbling around her ankles. Recent changes in her lifestyle....she was on vacation and did not keep her legs elevated as much as at home. Was also outside in partial sun but I see no sunburn. What could the bubbling be from?