Hello everyone.
I'm Mercedes.
I'm 17 years old and I'm in a weird situation..
Well.
(I'm bisexual)
I just got to see my girlfriend yesterday.
We haven't seen each other for a month in a half.
She hugged me very tight when she saw me and I felt very happy until she met this guy.
He gave her his yin&yang necklace..
Then he asked me if she had a boyfriend and I didn't answer.
But now her friends told her that he likes her and when I came out of where I was at she just said can I be with you both?. Why?. I don't understand.
What should I do?.

Give her your honest answer, is what you should do.

If you don't want to share her with another person, that's not selfish at all! That's what most people want, a 1on1 relationship. If she is not willing to do that for you then leave her, you will find someone else. Good luck!

Don't let her confuse you. Don't let her mess with your mind. Tell her YOUR honest opinion about the situation. Tell her how YOU feel about her. Stand by your feelings and values.

That would put the ball in her court and let her decide what she wants to do with the situation.