Hmm. Maybe my term for best friends is a tad different.
My best friends take a while for me to really consider them a "best" friend. At the point I call them best friend, they pretty much know me like the back of their hand, have welcomed themselves in to my house with out knocking, raided my fridge with out asking, and have gotten in trouble with me in some way shape or form. jkjkjk.... but along the lines... I guess i just feel like 5 months is a quick amount of time to consider someone your best friend? But it's different for everyone i suppose so .... that's irrelevant.
But based on everything else... it seems more like... a relationship thing... I know you weren't together... but for her to cut ties to so many things to you when her other friend is back in the picture seems a little shallow tbh. It'd be one thing if the other guy was her boyfriend, then i'd understand... so I'm assuming maybe she likes this person a little more than a friend? idk... I just think if you're friends with someone... and if someone was friends with you, best or not, they wouldn't feel entitled to put a limit on your other friendships unless they had jealousy issues which is more so being selfish than a real friend.
so i say do what Zeus/Zess says, too.
But either way... I'm glad you were able to work something out and I hope it stays that way for you!