AITA for talking badly about my friend of 10 years?

Yes, I know the title doesn’t make me seem like a great person, but I just need to get this off my chest, and figure out if I was really in the wrong.

My friend. Hell, my best friend. Let’s call her Kara.

Me and Kara have been attached at the hip for a decade. She’s always been there for me, I’ve always been there for her. We do everything together. But these past couple months, I feel like she’s been hiding something from me. Something she doesn’t want me to know.

For example, me, Kara, our other friend, and my sister all went for brunch about 2 months ago. Everything started normal, completely fine. That was until Kara had to step out to take a phone call. Obviously, there’s nothing bad about that, but when she came back in, she was physically shaking. Not because of the cold, out of fear. I could see it on her face. We all asked her if she was okay and if she needed to leave, but she started getting really defensive about it and said everything was fine. I didn’t try to get anything out of her, because it is her own business. But after that particular outing, she just doesn’t seem the same. Especially when around my significant other.

When around my boyfriend, she has so much anxiety, and I can tell my boyfriend does, too. I’ve never asked either one of them about it. But they act like they’re best friends. They’re both always together, always talking, etc. But it was never like this before. Only after she got that phone call, which is extremely weird to me. They even got asked if they were dating, which I think just sent me over the edge.

I was with my sister, and I just started going off about Kara. I called her a lying ass bitch, and a lot of other things I’d… rather not say. Anytime I even bring him up, she gets fucking anxious about it??? It could literally be something about how he shit his pants and she’d be all scared.

I’m just worried that theres something more going on between them. I’m really trying to trust both of them here, but I just can’t. Please help!!!