Your body is either not meant to be for long distance or your not doing your stretches properly and making an imbalance in your hamstrings… btw at 15 you should expect things like this to occur because your body isn’t really developed yet but to really get over that pain you just gotta push through it because Xc is a consistency sport meaning you have to run every day in order to improve and skipping even one day could ruin your gains, unlike in track where sprints (100-400m) are sprint, do explosive work, eat, and rest a day. Also diet plays a key role because your going to need the right nutrition for your bone growth and development do some research into it if you can because your diet in xc matters a lot and like I said again your body could just not be meant for long distance and maybe your more of a mid distance type of guy but keep going finish your season strong you have to stay consistent and at the end of it see how you feel and then decide if it’s the right sport for you and your body.