Slink Aiss' mate S-J says:

Outrageous! I can't believe your friend would do that, and you say you're best friends?! That's not cool at all, Mary.

Now she's clearly upset you, so you should have a chat with her to let her know how you're feeling. That was a mean thing for her to do, but even as her best friend you can't dictate what she does.

You have to be aware that it's good (for both of you) to have other friends too. If you spend all your time with one person then it can be too much and it's easy to get on each other's nerves.

If you become too possessive with your friend then she may not want to be best friends with you anymore - why shouldn't she hang out with other people too?

Try asking her why she did that and telling her how it made you feel. Maybe she didn't realise what she'd done? I'm sure she wouldn't hurt you intentionally.

After talking to her, also try to spend some time with your other friends. That way you get to have loads of fun and, if you guys ever do have a falling out (which I hope you won't), you'll have other friends you can count on too.

S-J
xxx