I Have A Torn Meniscus (inside The Knee Cap) And

I have a torn meniscus (inside the knee cap) and had an MRI last month. There are a couple small pieces of torn cartilage , floating they called it, in the back of the knee. That didn't bother me until the last week and I've noticed a swollen lump about the size of a large marble on the back of my knee. It swells more in the day and feels tight when I walk. I am scheduled for surgery the end of May..... what is the swollen lump on the back? Does that sound like a blood clot or more fluid. thank you.