I checked my bf's phone and didn't like what I saw

I know I did the wrong thing; checking other people's phones is not correct, but I had suspicions about him talking to other girls.

I found a conversation with a girl he recently met while pursuing a master's degree. He never uses emojis with me, like NEVER, not even when we were just meeting, but he used them on every single message he sent this girl. Am I being paranoid or could he be interested in her?

For the record, it was this emoji "☺️".

I hate that I've found this. Now I feel like I can't even look at him because I'm so upset.