What Causes Multiple Knots On Shin And Foot?

I have multiple knots over both of my shins and one on the top right side of my right foot. I will have the knots at least a week before the bruises show up. They have been there for almost a month with no improvement. I have no memory of hitting these spots at any given time. I have had an xray done and the xray appeared to have normal marking to both the shins and the foot. But, they still will not go away. Any ideas on what this cold be?