I think it sounds like it's related to the football. It's a rough sport, and you are probably getting hit or maybe the equipment you have to wear is bothering you. I don't think, at your age, that the way you sleep should make it hurt like this, but check with a doctor.
If you sleep better on yourstomach there is no reason to jeopardize your sleep by 'getting used to' a new position, but while on your stomach try putting a pillow under your mid-section to help align your back n a more natural position and alleviate the stress of the spine flexion resulting from sleeping on your stomach. lack of sleep can cause more stress to your body and increase your pain level when your body is not well-rested enough. If you can comfortably sleep on your side bend your knees up to release the stress on your lower back, or just rest in this position when it has been bothering you. After you've played football, it seems like you need to take the time to properly stretch-out those muscles as it sounds like post-game you're muscles are cooling down and spasming. A neat and simple trick I have learned through many years of back problems is that when you experience the pain alternate using heat and ice in 10 minute intervals. So, heating pad for 10 minutes, then ice pack (bag of frozen peas works great) for 10 minutes. Continue to alternate for as long as you want. you don't want to go longer than 10-15 minutes for either since irritated muscles can actaully increase in spasm activity if you leave them for too long. The heat will help relax the muscle and slow the spasms and the cold will sooth and reduce inflammation. There's always anti-inflammatories like ibuprofin to help too, as long as you dont over-do it or else you're likely to irritate your stomach. Hope this helps you, baz09. You're too young to be affected by such nonsense. Get it under control now so you dont risk 'muscle-memory' causing life-long, medical intervention type issues.