My boyfriend lied to me, what do I do?

My boyfriend of 5 years (lived together for 4) lied to me about hanging out with a female coworker. I caught him in lying and confronted him about it. He was open and talked to me about it, and his excuse was that he didn't want me to be jealous. He had added her on snapchat and asked if she wanted to hang out. He told me that he wants to get to know her better as a friend since they get along so well as coworkers.

When I asked about what they talked about it was just normal stuff like movies, tv shows and books, and future plans (parties and such), but he didn't answer all my questions. He would just say ''I don't remember what I wrote'' and he didn't save the messages on snapchat.

I'm scared because I don't know what do to, am I overreacting about this? Why would he lie in the first place?

Updates
11 mo
Update: thank you for all the feedback!
A little more information: we've been having a rough couple of months, with a lot of confusion from his side what he wants out of life. I guess that's why I'm so upset that he lied about hanging out with another woman. He's been very open about this and told me if it hurt my feelings and made me upset, he wouldn't hang out with her outside of work until things between us was resolved or if I met her.
My boyfriend lied to me, what do I do?
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