advice on strict parents

my parents are really strict, especially my mom. i get anxious to ask her to go anywhere and i’m 15. i ask her if i can go out with my bf (which i don’t think she knows i’m in a relationship) and she yells/gets hesitant. i don’t understand where this is coming from since i’ve given her no reason not to trust me. i’m responsible, do well in school, and i don’t ever do anything to cause trouble. she let my sister do a lot more things when she was younger, so i don’t understand the disconnect.