A dull ache on the back of my calf?

I run cross country and i just went to my first summer practice. I haven't ran in a while, until yesterday. Today i noticed that i had an annoyingly dull ache on my calf, kinda under the knee. It doesn't feel like a normal cramp though, and ive had a history of muscles spasms in that area before, but this doesn't feel like that. I was scared it would be a blood clot, but considering im not on any medication and im only 14, It wouldn't make any sense. (would it???)