You can't get pregnant unless sperm gets inside your body, and obviously that will not happen from solo masturbation (in other words, if you are masturbating yourself alone and not with some guy's penis very near your vagina). Not having had a period for a year is not unusual for someone as young as yourself, many girls don't start having periods in the first place until after they are 14. Next time you go for a checkup with your family doctor, if your periods have still not come on, you could mention that they began and then stopped. Or, get an appointment with an ob-gyn (doctor who specializes in women's health, called a specialist in obstetrics and gynecology). That kind of doctor can answer all of your questions and will not be embarrassed or embarrassing.