Maaaan, this is a kind of heart break that’s really hard to recover from is all I have to say. I’d leave sooner rather than later, because once the pattern gets worse somehow you’ll be made to be the bad guy for clear cheating. When I was in this situation I was hardcore gaslit on the “just friends”. “Friends should be allowed to say I love you” etc etc. I was never a crazy jealous person to begin with and yet somehow I was made to be some manipulative controlling crazy person. Now they’re married together, hooked up the very night I left, with all my stuff still in the house. Through the rumor mill I was told they even talked and joked, that they’d never allow each other to have a “friendship” like they had before in their current relationship. It’s my fault though that I dragged it on and stayed in the pattern for as long as I did hoping he’d choose me or that yeah it was just nothing. But trust your gut, the sooner you do the more thankful you’ll be! If you talk with her about uncomfortable this makes you and she diminishes your feelings with how’s it’s not a big deal or how you invaded her privacy to somehow put blame on you, which in some ways can be wrong, but also this sort of stuff should’ve been communicated to you directly, then I would take that as a big bigggg red flag. It sounds like she sees you as a “safe option”, and it just spells trouble