Open relationship

I’m 27F in an open relationship with my boyfriend 25M. A while ago, we had a serious talk and agreed that if we sleep with other people, it should be one-time-only experiences. He clearly agreed to that boundary.

Recently, however, I found out he had sex with someone, and the timeline he gave me didn’t match what really happened. He claimed it happened much earlier, but that just doesn’t add up based on when we had our agreement and the kind of communication we had around that time. That alone already shook my trust.

And now, I noticed he recently followed a woman again whom he had something ongoing with before we got together. She had come up in past conversations because she was still around when we were just getting to know each other. Eventually, he stopped seeing her and even unfollowed her. But now – out of the blue – he follows her again?

I had seen her profile back then out of curiosity, but never said anything. Now she suddenly pops up again as a suggested profile, and I recognized her.

Taken together – the situation with the recent hookup, the weird timing, and now this follow – it’s feeling like a pattern: sneaky behavior, vague timelines, half-truths. The worst part? Just a short time ago, he told me he wasn’t even interested in others right now and that he’s fully fine with one-time-only encounters.

Honestly, I feel angry, hurt, and betrayed. But I also don’t know how to bring it up without sounding like I’ve been “checking up” on him, even though this isn’t about spying – it’s about honesty and respecting the agreements we made.

Have any of you been through something like this in an open relationship? How would you handle it?

tl;dr:
We’re in an open relationship with the agreement that outside encounters should be one-time-only. He recently slept with someone and lied about the timing. Now he’s also following a former fling again. I feel betrayed and confused, and I’m not sure how to bring it up without being dismissed as overreacting.