What Causes Bruise Of Knees,lower Legs And Upper Arms?

Hi. Recently I've notticed that my knees, lower legs and upper arms keep getting bruised, seemingly at random. Lately the only real exercise I've been doing is walking. During the day I do routine housework and computer work. I dont bump myself often, but I've taken to paying close attention to when and where. Apparently I dont bruise easily, because bruises rarely appear in these places. But other places, like right above my knees, look like old bananas. What could be the cause?