Have you ever had a good relationship with your mother. If you have you should tell her. Of course your mother is going to be upset if you are to young to be having sex. But some mothers gets upset no matter how old you are because they don't want to think of their little girls as young women. But I think your mom knows that you are having sex. And she gave you a chance to be honest with her. And you didn't tell her the truth. So she isn't going to change her mind about not letting you go over to your boyfriend's house when you are not on your period. But if you go and talk to her she might change her mind. I was 19 when I started having sex and I talked to my parents about. And they didn't get mad. Yes I know I was 19 but my parents told me not to have sex until I was married and I didn't. But you should talk to your mom about it. And the is the right thing to do.
I am going ask you something. How do you think you would react if you asked your daughter if you was having sex and she told you the same thing you told your mom. Wouldn't you want your daughter to talk to you about it. Who knows your mom may carry you to get bc. If you are not already on it.
I hope I helped some. And I hope you can make sense of it.