Bf venting to another female..

So my bf of 8 years and I have been having some differences lately. Well I just he was talking to this girl. She had commented on one of his stories “you look sad and down” and they stemmed the conversation from there. He said he was a bit sad and having problems and such. Which yes he’s allowed to have female friends but the extent of venting to her instead of me was a little heartbreaking and I feel like the line was crossed big time. He says it wasn’t a big deal and that I can’t be the only he runs to for problems. Idk

EDIT: this isn’t a girl he’s know for awhile she’s someone I’ve never heard of before… like I said he’s entitled to having opposite sex friendships but it’s weird that I don’t even know who she is or have ever heard of her before