how do I tell my mom I'm having sex!!!????

Autumn
hello I am 16 years old, almost 17, and me and my boyfriend have been together for a year and 4 minths. my mom has recently been asking me if I'm having sex and I lied and told her I wasnt. she made me an appointment for BC and I go in December . however I feel really bad and guilty because I've been lying. someone please help me !!!!?