A Secret Meeting Outside Our Relationship Rules

For weeks now, Alex and I have been in an open relationship where we agreed to check in with each other before seeing someone new. Last night, however, I found myself at a party talking with a guy named Jamie. We had flirted briefly earlier in the week, and something about his confidence just drew me in. Without checking with Alex first as I always do, I ended up kissing him under the stars, our lips felt so right together. It was a split-second decision but also one that feels heavy now. Why did I feel the need to slip outside our boundaries? What does this mean for us?