My best friend lets me fuck her boyfriend

My best friend knows how I feel about her boyfriend. I don't even try to hide it, and neither does he. We've flirted and slept together, and she's always been into it, even encouraged it, but I feel like something has changed. As her boyfriend and I get closer, I feel like she's becoming colder. Like she thinks I'm stealing him, and that's not true. I needed to make sure she knew.I managed to get alone with her and started a conversation. I was surprised when she said she was totally fine. I was shocked and grabbed her hand. I wasn't being flirtatious or provocative; I genuinely wanted to know her opinion. She told me, honestly, that she loved seeing us together. That she liked the intensity that arose between us during sex. She kept talking and started fidgeting in her chair. I realized she was getting aroused just talking about it!