Why does my boyfriend have a secret friend who is a girl?

I have been with my live in boyfriend for 7 years, up until 5 months ago , we had a great relationship. I trusted him with my life. He treated me good, spent most of his spare time with me, didn't lie, and was the best person I had in my life. But recently I have discovered he has been lying to me about this girl who is a "friend" . Now i'm not crazy he can have friends. But this girl I did not know about. I figured something was up when he started staying out later than normal, not picking up phone when I called, ignoring my texts...etc.I just had this really bad feeling. I found them in a car one night, and I basically lost it. Everything started to make sense. And it all came to me as soon as I saw her. He swears they are just friends and they were just hanging out....this was very late at night when he should have been home sleeping. He told me in the past he just talks to girls at work but never hangs out with any girls alone. Which I believed. After that night I chose to give him a change to prove to me I can still trust him. A week only a week past and I asked him if he had any contact with this girl, he said no, they are not friends anymore after that night I found them and I freaked out. I found them together again a week later. I have also discovered they have been hanging out for months. I'm confused because when he tells me he loves me and wants to marry me and have kids with me, I believe him. I've devoted 7 years of my life to this man, he is the only bf I've ever had. and I love him with everything I have. He swears that nothing has ever happened with her and that he would never cheat on me, he wouldn't want to risk losing everything we have together.Am I just being paranoid or is my sudden untrust valid?