Could you have injured the muscles there somehow? Something different about sleeping position? Have you been doing something new?
I also doubt it's entirely your fit. However a longer reach will change how your shoulder muscles work during a sprint. Are your shoulders relaxed and dropped? Is your back straight? What does your coach say?
I assume, being 14, that you don't have much time on a racing bike yet? Meaning years. Neck strength takes time to develop. Neck pain is one of the commonest complaints of riders who are ramping up their time on the bike. You could do stretching and strengthening exercises. I used to let my neck go completely slack, and then while standing or sitting, let my head flop around 360°, a number of revolutions. It's hard to get the muscles to completely let go. Football players do neck exercises where they put a strap across the back of the head and then use it to raise a weight. I'm not sure that does any good for your problem, though.
You could certainly try a shorter stem. That's a cheap thing to try. Very little down side. If you have pain every time, then you're irritating your neck every time, which won't lend itself to healing. Best try to get it fixed.