My Knee Is Swollen Very Badly...has Been For At Least

My knee is swollen very badly...has been for at least 2 to 3 weeks....went to my doctor they did blood work and CT scan....tests really tell much dr thinks it might be coming from my lower back or hip....cant sit down or drive to long the pain is excruciating.when I sit ...my knee is 3 time the normal siize....you have any ideas what could be going on....