Am I being paranoid?

Every time I'm with my boyfriend and he goes into his gallery or replies to anybody he tilts his phone so I won't see. It's honestly really bothering me a lot since I have a feeling he's cheating on me but never been able to prove it.

Once he showed me something in his gallery and i accidentally scrolled farther than I was suppose to and I saw a photo a woman sent him (sexting him.) He showed me the conversation in which he just replied with an emoji and that was the end of that. I probably should have left but I didnt... That was 3-4months into our relationship. We now have 1 yr 6 months. I'm still not over it. And when he tilts his phone I get extremely paranoid that he's hiding photos like this... He has changed a lot since the begging of our relationship but part of me is always unsure about his loyalty...