What Could It Be If Purple Patches Are Developed On Leg?

Hi....my mother has developed some purple patches on her left leg, large spot behind n side of knee cap(this particular patch is darker, looks more black than purple) and some spots on the entire length of the leg. In addition to that the leg is swollen slightly from knee downwards including the ankle. She hasn't eaten anything that can be suspected as causing any allergic reaction. It has been two days.
Thanks in advance...