Help! Why did my boyfriend lie about going to the beach with another girl?

My boyfriend (29 years old) was a real player before we got together, and had lots of friend with benefits, including me. So I'm always a bit paranoid on the inside, but have NEVER expressed jealousy to him ever.

He's really open about what he's doing during the day, and who he hangs out with, but he's secretly hanging out with this specific, very hot girl. Imagine blonde, big fake lips etc.

He has only mentioned this girl to me once, after he ran into her at his gym, because he told me he met an old female friend, who told him that she's having an affair with his old friend who is married. Basically it was more a story about some gossip about his old male friend. Such morals she has...

Since then I have been looking at her instagram profile sometimes, because she's always in my suggested friends everywhere.

Yesterday she posted a story while I was at work, and it was a selfie of her lying on the beach, and my boyfriends bag and hair was halfway in the picture.
When I got home, I asked him casually what he had been up to that day, and he told me he worked out, went to get a bike fixed etc. nothing about the girl.

Then today , as he normally does, he told me he would meet up for at coffee with his recent female friend (who he doesn't hide from me), and he updated me about whats going on in her life - so it's only the hot girl that he keeps secret.

Is he straight out cheating, or is he just afraid that I might get jealoux of this girl, and decide to just keep their friendship from me?

Updates
1 y
Update: he asked me twice if "is something wrong?" because he could feel it on me. The day after he had deep cleaned that apartment and made me dinner. Clearly feels guilty, but for not telling me, or for cheating hmm?
Updates
1 y
He has seriously been a better boyfriend ever since... weird
Help! Why did my boyfriend lie about going to the beach with another girl?
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