the best way to get all of your answers is to call your doctor for a consultation.
you have a right to stop useing this if you dont want to and birth controll can cause differant side effects for different people so its hard to say how your body will cope with coming off the injection you may be lucky and get your regular cycle back right away... but if you do stop taking this form of birth controll and have intercourse theres a likely chance you could get caught pregnant to prevent this you can allways talk to a doctor in private about some differant birth controll that suits you. good luck :)
Why exactly do you want of the shot?
But yes some women can bleed for a month or more anytime a birth control is stopped from being used because of the hormone change in your body. Now if you want off the shot that is your call because it is your body however some states and territories do leave it up to the parent when dealing with minors. Next time you go to the Dr request to speak to her without your mom in the room and ask her about it if you don't feel comfortable asking in front of your mom.