AITAH for feeling some type of way about his coworker??

My boyfriend and I have been together for over a year and a half now, which doesn’t sound long. However, we have been best friends for several years prior to our relationship. Earlier this year, I went through his phone and found he was messaging a female coworker.
She was blowing up his phone with strings of emojis, letting him know she when she was drunk, he asked her what days he should take off of work. That was it. No cheating, no flirting (on his end), but entertaining the conversation, and asking her what days he should take off was enough to make me feel extremely uncomfortable.
We argued, and it boiled down to him no longer communicating with her outside of work, which I was content with. He then proceeded to delete the messages and change his phone passcode. I get it, I betrayed his trust, so I did my absolute best to let that slide.
I have now found out that he has added her on Snapchat. He “wasn’t thinking,” when accepted her on there. They haven’t spoken outside of work still, but I can’t help to feel betrayed.
He doesn’t seem to understand why I am so upset, and it’s genuinely starting to make me feel crazy. I don’t want to be obsessive, or controlling by any means. He does have other female friends, and I have male friends, but I just get that gut feeling that she’s into him, and he insists that it’s not like that.
So again, AITA??