Did she cheated on me? Need help

I’m 18, and I’ve been with my girlfriend for 4 months now. She’s really nice, and from the very beginning she showed me that she only wants me. She was always the one asking when we would see each other next, she unfollowed every guy on Instagram, and she did everything she could to prove that she only wants to be with me.

But in the beginning we fought a lot, mostly because she didn’t trust me. She couldn’t believe that I was serious about her and thought I was looking for someone better. She was overthinking all the time. During those fights I tried to give her more and more security. I always told her how serious I was about us, that I never wanted to hurt her feelings, and that I wanted to work things out. Over time she started trusting me more, and the fights became less frequent.

Then, about 4 weeks ago, I went on holiday. That’s when we had our biggest fight ever, so bad that we were close to breaking up. We both made mistakes that damaged our trust (but not cheating). When I came back, we forgave each other and had deep talks about how we felt.

Still, I can’t shake this gut feeling that she might have done something terrible. It’s not because of one specific mistake, it’s just new. After all the fights we went through, and after I gave her so much security, her behavior changed in a way that feels strange to me. Normally she’s always been jealous, and no matter how many deep talks we had, that part of her never really disappeared. But recently, two girls DM’d me on Instagram, and even though my profile picture is of me and my girlfriend, she just laughed it off when she saw it on my phone. We simply continued our conversation as if nothing happened. That’s unusual, normally she would’ve reacted with an attitude. On top of that, she now avoids arguments or fights completely, something she never used to do.

Another thing I’ve noticed is that she brings her phone with her more often when she goes to the bathroom. Sometimes she leaves it on the bed, but usually only if she knows she’ll be right back. Before, she would always leave it behind, even when she went to take a shower.

Our intimacy has also dropped. We don’t have deep conversations anymore, and sexually things are less than they used to be.

I don’t know if I’m overthinking, but my gut keeps telling me that she did something. At the same time, I wonder if maybe it’s just because we’ve been spending so much more time together. In the beginning we saw each other once or twice a week. For the past three weeks, it’s been three to four times a week, sometimes even more.

Part of me also thinks maybe it’s a lack of attraction, because I gave her so much love, security, and attention that I’m no longer a mystery to her.

It’s breaking me. Sometimes during the day, I convince myself that she just trusts me more now, that’s why she isn’t overreacting. But then I think, if that’s true, why are there all these other changes?

TL;DR
Right now, I feel like I’m at my lowest