I discovered that my girlfriend has a past with her best friend and roommate.

I, a 25-year-old female, and my girlfriend, also 25, have been together for over a year, and it's been incredible. We've shared countless adventures and I've never felt more like myself than when I'm with her. Recently, we've been discussing the possibility of moving in together and our future plans. My trust and love for her run deep, and the thought of the life we could create together excites me. However, this weekend, I discovered something that’s been troubling me, and I'm seeking advice on how to navigate it. To provide some background, my girlfriend has a best friend, whom I'll refer to as S, a 25-year-old female who lives with her. They've been best friends for years, and I've grown close to S since we started dating. I was aware of their close bond, like family, so I never thought much about it. That changed this weekend. We were enjoying a cozy evening, cooking dinner, sipping wine, and listening to music. While scrolling through our phones, we stumbled upon an old photo of my girlfriend and S that seemed quite intimate, depicting a romantic dinner together. In jest, I remarked, "Ooh, romantic. Did you guys kiss or something?" She seemed flustered and kept insisting, "Ew, no," but never clearly said no. This struck me as odd, so I teasingly said, "You’re acting like you hooked up or something." Again, she became flustered without definitively denying it. I felt unsettled for the rest of the evening but decided not to press further. After watching a movie, we went to bed, but the following morning, I brought it up again. She still wouldn’t provide a direct answer, suggesting we discuss it later. At that moment, I was anxious and confused. Eventually, she confessed that a few years ago, when they first met, they had hooked up a few times, both regretting it and wishing it hadn't happened. A few times? Naturally, I was upset, especially since it felt like I had to pry the truth out of her after our year together. I would have preferred to know from the start, no matter how uncomfortable it might have been. We talked it over, and she apologized for not disclosing this sooner, explaining that she didn’t want to create tension or risk losing me. However, now that I know, I feel it's important to establish some boundaries. For instance, their shared bed when they travel together makes me extremely uncomfortable. She couldn’t understand why this bothered me, asserting they’re just friends and would never act on anything. But despite knowing they wouldn't now, the thought of her sleeping next to someone she had once been intimate with disturbs me. This situation has prompted me to reevaluate many discussions I've had with both my girlfriend and S over the past year. I'm left questioning things that I previously took for granted. It's a strange predicament I never expected to find myself in.