Why is my girlfriend (F20) upset I (M21) talked to girls after our first encounter ?

So I’ve had a crush on my girlfriend ever since I transferred schools, but I never talked to her—just saw her in classes and thought she was stunning. Life went on, and I talked to other girls because I never really saw her beyond that. We eventually met at a formal, hit it off, and ended up sleeping together that night. I walked her home, gave her some of my clothes because it was cold out, and everything felt great.

When I went back, I dropped off an umbrella for another girl I had been talking to—she lived in the same building. Fast forward to now, my girlfriend brought this up because apparently she went through my phone and saw that I had dropped off the umbrella and also saw a text where I asked another girl what our status was.

She said my morals were all wrong in that situation. But for context, this all happened two months before we were officially exclusive.

She also says I’m awful because one of the girls I was talking to was 18 while I was 20, and that’s morally wrong—even though we’re both in college.