I would definately go to a doctor, if you haven't all ready. I'm no doctor, but I would definitely be concerned about a clot waht the lumb behind your knee.
I Have Fluid Under My Knee Cap,but I Also Have A Lump Behind My Knee Where It Bends.This Morning When I Wok Up My Ankle Was Swollen. Can You Help?
If you've recently had surgery on your knee, this could be post-op fluid which can be easily removed. However you should consult a doctor if this fluid produces pain or stiffness in the joint. You should also consult a doctor if the fluid moves down the leg as this could be a clot. An Ultra-Sound should be able to tell if you have a clot. A tear in the meniscus can cause fluid to drain into a number of areas of the knee. This will cause pain and should be surgically repaired. Another cause of the fluid sac could also be a Bakers Cyst. In either case the source of the fluid needs to be repaired surgically.