Talking to the parents??

Okay so me and my mother are really close, like super duper close.. Me and my boyfriend recently started doing small sexual acts. I feel the need to tell my mom and talk to her about it, I'm really afraid of how she will react because she told me if anything changes I'm going on the pill. But the thing is I don't want sex, I have no interest in it at my age(17) I've made a vow with myself not too have sex until after highschool to avoid the drama that usually follows.. How do I bring it up? And has someone else had an occurrence like this and is able to share what happened? I feel like she would be understandable because it's recently happened and I have been with my boyfriend for 5 months... Help please?