Severe pain in calf muscle and a lot of fluid in knee what could cause it?

Injury: Fluid around the knee can be caused from trauma and/or injury. The calf may pain may be a resultant spasm from the injury. I would recommend following up with your pcp and/or pain specialist on this.

Many causes.: it can be arthritis, a torn ligament or tendon, a meniscal tear , or a loose body, just to name a few possibilities. See an orthopedic knee specialist for evaluation.