What Causes Puss Filled Red Spots On Knee?

OK, I have this small red spot 1/4" diameter with a puss filled spot in the middle above my knee. It's the only one and usually only get one ant a time. I've had them on my legs, and hands, Looks like a flat pimple. I saw a dermatologist a week ago when of course there were no spots. All he said was he didn't think it was syphylis. What could it be?