painless lump below knee (Read 632 times)

I'm looking for some thoughts. I suddenly (within less than 24 hours) developed a fluid-filled lump below my right knee - just below the joint. It doesn't hurt and I haven't injured the knee. My shoes are new and I use orthotic inserts. I have been running for years and have never encountered anything like this. The only change in running was to add 1 mile to my 4-mile run (building back up after a stress fracture) on the day before the lump appeared. I had been at 4 miles for several weeks already.