Back of knee is swollen, painful, and red. its been like this for a month. it hurts more when im out in the cold. its a big red and purple splotch. ?

Baker's cyst?: The most common cause of swelling behind the knee is a fluid collection called a Baker's cyst. (Google it for more info.) Soreness and swelling suggest it might have ruptured or is leaking, or possibly infected but that's less likely. See a doctor; you might need surgery. If you have fever or pain is severe, don't delay: get care ASAP.