I am also 15 and a fast bowler 70mph+, i have weak pains in my back in the day after in the day after bowling, the pain is on both sides of my spine in the muscles, it isn't that painful, i am just wounder if it might be a problem. As in the winter i completely changes my bowling action as i fell over a lot when bowling. I sometimes get dont get pains the day after this is when i dont slip and fall, but some games my front foot slips and i fall over, this is when i get the pain.