Water On The Knee?

I've been sensing a bit of an achy feeling in my knees lately. Nothing I'd call pain, just a small aching occasionally.

Tonight after riding around the block and around the house in the grass I noticed a lump on the side of my left knee. I pushed on it and it was soft and depressed easily, sort of like pushing on a puffed out cheek.

I used the google and I think it's "water on the knee". It sounds like overuse can cause this. I'll have to read more about it, but icing can help, so I started with that. Of course, giving my knees a rest is a good idea.

So I'm hanging up my unicycle for about a week.