What Causes A Mass In Knee After Total Knee Replacement?

I tore my meniscus about 3 months ago, had a MRI, showed I have a severe tear and a mass in my knee also I had a total knee replacement on left knee about 6 months ago, my right knee never bothered me prior to said surgery., can this be a case of overcompensation from favoring the right knee when the left knee was healing after surgery.