I have this rash behind me knee that I have had for a year a doctor told me it was an allergic reaction and would go away. A year later its still there, sometimes its itchy and raised sometimes it looks like it goes away but still looks bruised. Some info about me  female 30 years old, no other known illness, I work out regularly and take a multivitamin.
Looks like you have an allergic reaction or an edematous patch behind your knee. Such lesions are more irritation in nature though less harmful. First you have to avoid touching the area too much and avoid itching. Continous itching tends to increase the edematous changes and increases the lesion. You can apply some topical moisturizer or oil over the affected area daily. This will tend to reduce the itching sensation.Also, you will need to use some topical steroid cream to heal the lesion completely. Take oral antihistamines along if the itching is unbearable. Hope it helps Chat Doctor.