Hi My 11 Year Old Son Keeps Saying His Knee

Hi my 11 year old son keeps saying his knee hurts (only one knee ) it's actually at the side of his knee on the outer side , there is no redness or bruising but when he bends it a large lump appears, the more he bends it the more the lump seems to stick out, it is a large roundish lump and I've looked at his other knee when bent and there is no lump, Jst on the one he's complaining with , my eldest son says this is just a muscle popping out ? If this is true then why is it sore and why only one knee? This has been going on for over a month now , any ideas what it could be ?