this isn't period but i need help

A few months ago my right lower back almost by my hip started hurting occasionally, but it was only when i ran, sat for about 15 minutes, or stood for a while. I dont know what happened but im an athlete so i get aches and pains and didn't think anthing of it. Recently its been alot more frequently that it happend and im starting to get worried. I think i nees to go to the doctor but im underage and can't schedule an appointment myself. I want to ask my mom but shes really tough and when i come to her for pain the usually says "youll be fine" so im nervous that if i ask her she will do the same thing and think im just being dramatic. I just dont know how to ask her without her doing that because its a very bad pain and i havn't told anyone about it. How can i ask her and her take me totally serously?