I had a large lump form on the back of my left knee last year, since then, it has doubled in length and mass. I went and got an ultrasound and I was told I have a baker's cyst. After a few more weeks, they checked it again, and told me it was large, and should be drained. However, the fluid was purely blood, which I was told is not normal. They referred me to get an MRI scan and said there was white blood cells found in the fluid that was drained. What could I be dealing with here?