slept with my best friends boyfriend

(They had a fight and she shut me out. So he came to my place to see if she was there and I said no and I was talkin to him in my room and they fought about him dropping out of School but she thinks it's a bad idea. And I said he should do what he thinks is best. And then he kissed me and we slept together. I feel bad obviously) 

* EDIT *

You guys are saying should of said no not here. But he was really upset and we are pretty close friends all 3 of us so... I couldn't of just shut the door on him. 

*edit again..*

We were not flirting. I gave him a hug and then he kissed me and I kissed him back and things happened

We've been best friends since we were 2 years old. I'm so scared. 

Edit* I told her... she didn't care cause she was planning on breaking up with him for ages. She was happy I did it tbh. They have been dating for 1 year and 5 months