XC/Track Runner...yet overweight?

I'm 16 years old female..and this is my second year on both xc/track. Right now its post track season training, and I feel really uncomfortable about my weight. I'm 5"3 and 150 pounds... and I guess I'm doing okay for a runner.... 2:50 on the 800M 24:37 on the 5k, and 69 seconds on the 4x400M relay.

My friends say I just have muscle, but I think I'm fat. I checked online and my BMI indicated that I was overweight. My legs are very muscle-y but my arms and stomach area need some work. Am I really overweight? Can you give me some ideas about what exercises I could do to help those trouble areas...? Any help would be amazing.