My(29f) brother?s(23m) girlfriend(23f) suddenly hates me.

They have been together for around three years off and on. At first she seemed to like me. She even suggested hanging out together on a few occasions. We would get our nails done, play mini golf, have a sauna days, etc.. I was really excited because I don?t have any sisters, and I thought she was going to be a great sister-in-law if they ever got married.
Even when they were in their ?off? stages, she kept talking to me and we remained friends. We were even able to joke lightheartedly about my brother without stepping on any toes.
Suddenly, a few weeks back she stopped talking to me. She totally ghosted, not responding to any of my texts, and never initiating anything. I asked my brother if something had happened between the two of them, but he said they were still dating, and were really serious about each other. I asked him if he knew why she wouldn?t talk to me, and he seemed to want to avoid the subject, just saying, ?I don?t know? a lot and brushing it off.
I?ve been wracking my brain about anything I could have done to upset her. I?ve reread text messages and can?t find anything even hinting at this sudden change.
I will admit, I went over to his house once when I knew she was there to try and act like nothing had happened, but as soon as she saw me, she **** down and kept looking at her phone. I?m pretty sure she was texting my brother who also looked really uncomfortable, (but denied it when I asked him about it later). She then excused herself to the bathroom and kept leaving the room for long periods of time while I was there. Am I overthinking this? I?m so confused.
I?m completely out of ideas, and maybe someone else might have some insight if something like this ever happened to them?
TLDR: boyfriend?s girlfriend changed from liking me to hating me for no apparent reason at all, and my brother won?t tell me why either.


Source.