Effusion means that there was "water" accumulation in the joint, and it usually signify that there was some damage in the internal structures of the knee - and that's why the knee was "swollen". I suspect you may have damanged your meniscus as squash is a sport that involes a lot of changing direction under high speed and high force, the meniscus would usually be under a lot of pressure and can be damaged during an intense squash game.
I'd highly recommend you to seek treatment with an Orthopaedic Surgeon and we would do a MRI scan for you to thoroughly investigate the cause of the effusion and the pain and then we can advise you further.
Hope that it clear some doubts for you.
Cheers,
Dr. Henry Chan
