How To Treat Bruise On Knee?

Hi I had a very slight itch on my kneecap at the side while hanging out the washing I scratched it twice through my jeans and it went once sitting down as I crossed my other leg over the leg that had itched it was painful I rolled up my trouser leg to see my vein about an inch long sticking up I rubbed it gently for a few seconds with my two fingers the flat of them and a while later it was really sore the vein had gone but a bruise the size of a fifty pence peace had appeared with no pain that was four days ago the bruising is going but still there