How Should I Tell My Mom

Recently I lost my virginity to my boyfriend it wasn't planned it was all out of love but now I don't know how to tell my mom. I usually tell her everything no matter what but I feel like if I tell her she's going to think we're going to do it all the time everytime we hangout at each other's house when that's not the case. I'm 18 but I still live with her and live by her rules. what should I do wait a few months or tell her right away? Tell her in person or write her a letter?