I have a large mass growing in my knee i didn't hurt it or anything?

Knee swelling: It sounds like you need to visit your physician soon. Sometimes the knee can become filled with fluid for many reasons, and it feels like a mass. Your doctor will determine whether to send you to an Orthopaedic Surgeon, who can do imaging of your knee and also do arthroscopic surgery to visualize whatever findings you have on imaging studies, take a biopsy, or recommend other interventions.