Should I tell my mom...

Should I tell my mom I'm having sex? Or shouldn't i? I feel as though it's only between you and the other person... Right? But how do I tell her? I feel like that's so awkward! She's had the talk or whatever with me and was open to buying me condoms but how am I supposed to tell her I'm having sex? Is she just supposed to know somehow? How did you guys come about that?