First Relationship - Is she cheating?

So this is my first relationship, about 3 months. I'm 21, she's 19 and we both go to the same college. We pretty much spend everyday together. She sleeps over every night and comes throughout the day. I slept with this girl throughout one fall semester. She told me she liked me and I said i didn't want to be anything. Then the next semester I found out she was sleeping with other people, so I told her I liked her and we started something.

My first mistake I think was going through her phone early in the relationship and saw all the random guys...Tinder, messages facebook, snapchat....she talked to. From that I found out she slept around quite a bit but I'm not one to judge but obviously it still bothers me a little.

Theres a couple things that have come up to make me suspicious. First there was a day party she went to, some guy asked if she wanted "a tour of the house". She went inside with him, she said they didn't do anything, that she just talked to other people. She went with a friend that always ditches her at parties so that was her excuse of accepting the invitation of going inside...to get back at her.

She lied to me about taking birth control for a period of time. One time she had a pregnancy scare and didn't tell me, said it was her fault so she would get it taken care of by herself...abortion.
She went out one night in a pretty skimpy shirt, kind of see through, and no bra. I showed up and she didn't seem really thrilled to see me.

There is one guy she slept with before we got serious. He is in one of her classes. Once she took a quiz with him while I was at work. I talked to her about it and said she can't talk to him anymore. She asked him what the hw was a couple weeks later. One time he texted her at 1 in the morning on a day she told me she couldn't sleep over but ended up falling asleep at my place. All the text said was "Do we need it for something." and the txt feed was just about hw. So idk if I'm just being paranoid or he knows about me and not to txt certain things.

One afternoon he texted her about a party. She responded "Litt" and deleted her message. That night she came over and said she wanted to go get coffee and it was like 10 at night. Not sure why she needed coffee....probably to sneak out to the party. But the guy texted her "When do you want me to pick you up". And we both saw it at the same time and she stormed out of the room angry. That night we argued for like an hour. She said she freaked out because she knew I would and I would continue to never trust her. She still slept over though. I even texted this kid and he said "no ones worried about your girl bro. Your good."

This is my first relationship so cut me some slack. I tell her to her face that I don't trust her and have accused her of doing things. So not sure if girls get really pissed at that. I'm just not a trusting person.