My Best Friend Just Told Me She Had Sex!

So my best friend just Snapchatted me saying that she had sex (it was her first time). Why do I feel an ache in my stomach?! I'm not in love with her or anything like that but I'm confused at what this feeling is. Is it the feeling of being left behind? She met this guy online a little while ago and now they want to be serious. I've never been in a relationship before or had sex... is it normal to feel this way when your friend moves into a different ballpark of life?