Hi otter8,
This isn't uncommon! I would chalk it up to the adrenaline, and to certain muscles flexing. In some contact sports, like wrestling, this is quite normal, and is usually completely non-sexual. Also, you're currently going through puberty, right? Random and frequent erections are quite common during that stage, and they can often happen for no particular reason, as
this article says: "many people associate getting an erection with being ready for sex or ready to feel sexual pleasure. Truth is, erections can and often do happen quite a few times every day for no particular reason, and certainly not because anything sexual is going on. It just happens. If you're not interested in any kind of sex at the moment, including masturbation, you can just ignore it, and it'll go away". Also, strenuous physical activity like sports, when carried on for longer than a few minutes, usually makes erections go down, because the blood has to flow to your other muscles that you're using. If possible, it's best to just push through and keep playing until that happens. Is that feasible for you?