Ignore the BMI chart. It was never meant as a measure of health, and it's pretty unreliable. All it does is take your height and weight and put it in a formula (some advanced versions also take body measurements); the problem with that is it doesn't take into account that 1)muscle weighs more than fat and 2)someone can be "overweight" but actually have very little body fat if they have a lot of muscle (think football players or serious weightlifters).
While it's possible that your weight might have something to do with your pain, usually weight will contribute to joint pain. Yours sounds more like muscle pain, which is probably from the running. Pulled muscle, maybe?
It could also be bone pain though. If you're 15, you're probably still growing at least a little bit, so it's possible it's pain from the growing process.
Try ice and some ibuprofen (and rest, if you can). If it gets worse or doesn't start getting better, see your doctor. smile