DISCLAIMER: I am set to meet with a psychiatrist about BPD. Outside of that I have kept long-term relationships in the past with little-no flare-ups. I'm writing this here because I don't feel like I'm thinking too clearly about the situation and need outside opinions.

Throwaway account because friends follow my main account.

I (18F) have been with my current boyfriend (18NB) for about 4 months, and we have known each other throughout our childhood. Yesterday night, I realized that he had been super quiet throughout the day despite allegedly doing nothing all day, rarely responding to anything I sent him and leaving me on read for long periods of time. This is really unlike how he usually is, as he usually texts/responds constantly (except for when at work obviously). At the end of the day, he texted me, telling me that his childhood best friend (18F) asked him to her sorority formal and wondering if I would be okay with it.

My boyfriend, bf, the best friend who I'll call T, and I were a very close-knit friend group throughout our entire childhood. Long sleepovers, game nights, camping, haning out after school, etc. as a trio were all things we did at least once a week. Since college, we've sort of drifted apart from T but I'd still consider us pretty close. Throughout my entire time knowing T, I would never have suspected her and my bf to have even considered a relationship. They acted much more like siblings than anything else, and the three of us had a very "TMI doesn't exist" type of trip relationship, so I'm sure if T ever thought of bf that way, I would've learned years ago. Another thing is that T is polygamous and constantly hopped from relationship to relationship, which is pretty unlike bf. However, T has also dated some people w/o telling their poly/open relationship partners in the past, some people which I never would have guessed T to have feelings for.

I initially said yes. I just asked to be updated throughout the night because our childhood closeness convinced me that T would kiss her brother before she ever kissed bf, but I'm still really hurt by the idea that my bf would ever ask that. Again, I know that T and bf would probably be close that way, but I still kind of feel that even considering taking another girl, let alone asking me if he could take her, is rude in itself.

This could also be me projecting from having been cheated on before (another person), but I don't feel good about the situation, and I want to rescind my decision. I feel sick at the idea that he'll be dancing away with T while I'm most likely just laying alone in bed for the night.

I also feel like it would be an AH move to suddenly say "nevermind" to my original acceptance, especially since he has begun acting normal again ever since last night.

WIBTA?