Does Running Cause Painless Fuild Buildup In The Knee?

I am a 39y/o male who has had a meniscus repair on 10 March 2011. Post op has been very good and I finished a three week bout of physical therapy strong. I have continued my weight training and a increased my running after PT. This past week I increased my mileage to around 12 mile for the week - mostly in 3 miles workouts. This past Sat (June 11th) I noticed that my knee had some build up fluid in it. Not a lot of fluid but just enough to make the knee look slightly larger than my other knee. I currently have zero pain in the knee and joint - just a bit of fluid. I have never had fluid build-up in that knee since my surgery. Is this a case of overuse from ramping up my mileage or should I be concerned? Thanks.