Just because you are getting the Gardisal injection has nothing to do with being sexually active- isnt that the ovarian cancer vaccine shot? My female students aged 12-18 have all had the shot given to them at school. The majority of them have never had sex or are sexually active. If its the vaccine then its not about sex, so your mother doesnt need to know you are sexually active.
Hope that made sense, you have every right to privacy.