Boyfriend - trust issues

My boyfriend of 2 years and I have had trust issues recently. I never had a problem trusting him until he told me that he had cheated on his last girlfriend twice and never told her. This made me a little uncomfortable even though he was being honest by telling me. Recently I went out drinking with one of my best guy friends who ended up kissing me and telling me that he had feelings for me (which I don't return). I told my boyfriend about this a couple days later (right before thanksgiving), knowing that he wouldn't be happy about it. He told me that it was okay and we should just move forward; he said he still wanted me to come to Thanksgiving with him and his family. So we flew to his house for Thanksgiving, but the entire time it felt like he was acting weird and "faking" things a little, but he continued to say everything was fine and we wouldn't break up over this, he knew that it wasn't my fault. I also noticed he was texting some girl I had never met every day over the break. On the day we were supposed to leave, I looked at his phone and saw that he had told this girl that he wanted to break up with me and had no desire to kiss me anymore and she had agreed tell him that I was a "cheater." So after we got to the airport away from his family I told him I could tell he wanted to break up and if he couldn't do it, I would. He didn't argue and we ended up taking a break. A couple days later he told me he didn't want to break up permanently and he definitely wanted to get back together... just not yet. But he couldn't tell me why we couldn't get back together yet. Then after another few days we did get back together, but I notice that he is still texting this girl every day. I asked him if he told her that we had gotten back together, and he said that she never knew we were having problems, and he had never told her anything. He said all they ever talk about is class and studying, and he felt like I was interrogating him by asking if she knew we were having problems. I know a lot of this is background information, but I just want to know why he would lie about what he said to her? Should I be worried about this or just brush it off?