Am I being paranoid or is my step-dad lying to me?

My step-dad and I have not had the best relationship. I generally don't trust most of the things he does and our personalities tend to not mesh well. I try to get along with him for the sake of my mom because I love her more than anything. They got married when I was 15 and I'm now a couple months shy of 18. Every night my mom and I go for a walk together for an hour and a half or so. Tonight I left my unlocked iPhone on the charger in my room on my bed. I usually don't do this but I was in a hurry. My step-dad was home with my step brother and sister. I guess while I was gone he got the opportunity to get ahold of my phone. When I got back, I opened my phone to read my texts. I realized I didn't have any notifications which was odd because I was texting a couple of people when I left. I opened my phone and one of the messages my boyfriend had sent me had already been read. Since I have my read receipts on, I asked him when it was read and it was during the time I was gone. I then double clicked my home button and looked at the most recent things opened and it was my messages, videos, photos, and settings. I walked into my living room and announced aloud to my family that someone had been going through my phone. Assuming it was one of the kids, I was surprised when my step-dad came in my room to tell me it was him. He told me that he was looking through it to better understand my little brothers iPod 4. He said he accidentally opened my texts. Also, the videos was on my fourth page in a folder labeled "never used", so I don't understand why, if he was looking for settings which was on the first page, he was way back there. If he was really wanting to learn more about it, why didn't he just ask me to help my brother? I also don't think my mom had anything to do with it because I heard him explaining it to her too. I would just like to hear what others outside of the situation think. Would you go through someone else's phone to better understand another device?

He's lying. Maybe he was making sure you don't do anything wrong but videos and photos that's actually personal for him to look at

I think he's lying

It sounds like he is lying to you. You need to speak to your mom about that. You can also speak to him and tell him that you didn't appreciate it that he went through your private things. Tell him to ask you next time before he rummages through your phone. If he argues, it's because he knows that he did it.