Verified answer

To start with, since you're just 14 it's unlikely to be anything scary. It very likely is a simple "stitch" in your side. It's actually your spleen contracting. (Your spleen is an organ that is part of your lymphatic system, which tries to regulate your total lymphatic volume by acting a bit like a sponge.) Yes, your suspicion that it has to do with not exercising for a while is correct in that case. It should stop happening once you start exercising regularly again.

However, it's always a good idea to have regular medical checkups, even at a young age, and you should probably not rely only on Yahoo Answers for advice on matters regarding health. Definitely mention it to your coach, especially if it doesn't go away, and mention it to your parents, too, who might know better than you or your coach if there ought to be concern. This last advice goes for all health related questions, in any event.

it could be a cramp. try drinking more water a an hour-two hours before you run. But ask your coach also.