My knees hurt recently when i run, ( i'm in track,) they don't hurt a lot but they sometimes make my legs feel heavy ?

Overuse: What you described could be due to overuse. Depending on your age, your bone status may be a factor to consider is well. Chronic pounding from running track could overtime cause what you described. You may have actually bruised the bones. You should get a structural analysis of your spine. If you are a teenager, there are a few other bone issues to consider.