I have a small pea size lump under skin behind my knee on crease of back of leg. It has been itching. Now it is purple/red from me scratching it?

A little more info. If it is movable and just under the skin, it can b a small inclusion cysts not originating from the knee. Ur scratching has caused the changes u describe. U should c an ortho surgeon 2 have it evaluated & perhaps excision if u can't keep ur hands away from it. It may get infected & cause problems.