Don't pretend you didn't notice it, like by not mentioning your plans or pretending you didn't go out with anyone. This just makes things worse because she'll feel even more left out when she finds out, and then it gets way worse. It's also not cool to criticize her harshly or be defensive, because she's clearly insecure about your friendship.
Yeah, a conversation is necessary, but it has to be careful. Try to talk at a calm moment, not when she's complaining (because that gets too emotional). Explain that you love your friendship but that you also need other relationships and social activities, because that's actually healthy. Make it clear that this doesn't mean she's less important to you.
Also try to understand what's changing for her. Sometimes people get like this when they're insecure about friendship, when they're going through something tough, or when they realize life is different from when you guys were younger. Maybe she was expecting you'd keep doing everything together, you know? Then it's worth suggesting specific things you two will do together, so she sees that quality time actually exists. But make it clear that you're not going to stop going out with other people because that's not fair to you.