I (21M) cannot understand this woman (31F). What can be done?

Basically, I go to a gym where she is my trainer's girlfriend. We all became extremely good friends and we have kind of an older sister and younger brother kind of relationship. She is a very friendly and sweet lady and I want our relationship to stay as it is. However, this seems to be drifting away. There is a guy (20M) and she treated both of us fairly equally. However, since the last few days, this seems to be changing. She seems to prefer to talk to the (20M) guy and get wry of me for some reason. And it feels like we are drifting apart. Somehow and somewhat, it feels like she is awkward around me. It feels like it is all in my head but it hurts so much, I cannot put it into words. Asking her directly will put her off and make the situation worse (as fas as I think). Things are going horribly wrong. It feels like I said something and she didn't particularly like it, so she hid it. Wtf can be done? I want everything to be back as it was. What am I supposed to do in this situation? How to fix it? Can something be done at all?

I (21M) cannot understand this woman (31F). What can be done?
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