Pain below knee 3mos now. no injury. xray mri normal. now less pain + little bump what could it be?

Osgood Schlater's : In growing children it is a traction injury ( due to sports activity ) to tibial tuberosity apophysis. After the growth is completed, the pain subsides and ususally you are left with a bump.

Any # of things: This could be a number of things. Get evaluated by an orthopaedic surgeon. One in your area can be found at www.Aaos.Org.