Help! How do I handle my mom finding out i'm having ?

Ok, so i've been going out with my boyfriend for 1 year & 2 months. I'm 17 hes 19, but i was 16 when it happened and he was 18. We've been having for a little more than a year and my mom has been having suspicions. I never thought it was a big deal. One day my mom randomly wants to have me see a OB/GYN, and like literally almost 5 mins later, she sees a hickey, (I DIDN'T EVEN KNOW I HAD) on my neck! she starts yelling at me, saying im going to get pregnant, shes putting me on birth control, blah, blah blah! She's taking things over the top, (i think), and its bad enough she barely trusts me -even though she claims' she does. Now shes giving me those 'im so disappointed' in you looks and ignores me (which i don't mind), but how do i get her to understand, that im responsible?!??!!