Shins splints would probably manifest in the front of the lower leg, not the calf.
You may just have tight calves, and need to stretch them out a bit (I use a foam roller), do a google for calf stretches and calf strengthing excersices.
I get bad calf tightness and pain when I run over 12 miles, and that because my calves aren't strong enough.
Also, considering your age, I wouldn't put growing pains out of your mind either, I remember in my late teens getting shocking growning pains in my legs.
Could also be a gait issue, so if you aren't wearing supportive enough footwear, that will cause problems too.