What Could Cause A Lump On The Center Of The Knee Along With Some Stiffness While Walking?

Hi, Three hours ago I had a perfectly normal lef knee, went out to dinner and leaving the restaurant I noticed my knee was a little stiff , placed my hand on it to massage it and noticed a walnut size lump had formed in the center of my knee cap. It’s not painful and it’s pretty hard. What are your thoughts?