I felt a sting on back of leg just above the knee. now there is purple rash?

Insect bite: What you described suggests an insect bite. Unless the area of rash doubles in size, becomes more painful, you get a fever or the site bleeds, you may leave it alone to heal by itself. If any of the above happens, please see a doctor.

Reaction...: If there was a sting or bite the usual response is a skin reaction. Monitor the area and if the symptoms increase, the rash spreads and/or gets painful you will need to go to the ED as that could be a potential allergy. Get better soon!