What Causes Bruises On Knees?

hello, im 24 yrs old average weight for my small height off 5ft 1in. For over a year now i have been having bad pains in both my legs which can be limiting at times. this started to happen after i had an emergancy c section. Now every couple of weeks i get a dark purple bruise just above the top of my knee. Its the size of a pea, and it does not hurt like a bruise. I have not knocked it at all. Do you know what this could be?