Is my bf hiding something from me?

He has never told me his password, when I have asked. Years ago. We’ve been together 6 years. Anyways I forgot all about that since I never look at his phone and vice versa. I sent him a funny Snapchat right now, and I wanted to see his reaction as he opened it. He went to grab his phone and we were both looking and as he opened Snapchat he hides his phone and says “I want to see it alone” which doesn’t make sense because I was the one who sent him the funny Snapchat and I wanted to get his reaction. As soon as he did this it made me feel like a really weird feeling, like he’s hiding his Snapchat on purpose. He opened the Snapchat when I wasn’t looking but at that point I was already sad by his whole reaction. He didn’t notice that I was sad because I just fake smiled. When we first started being in a relationship years ago he gave me his password and I saw him messaging a girl. So now this makes me wonder if he’s still doing the same thing or how come he has to hide his phone from me?