It was a spur-of-the-moment thing. Completely unplanned. You and Michonne were in Alexandria, in your house. Things were stressful. Food was short and supplies were short, everyone was on edge, especially with the looming threat of the Wolves coming back.

The two of you were on your couch, having just completed a talk. You comforted Michonne, more than anyone, about everything, and she ended up kissing you.

Rick was looking for his girlfriend, Michonne, he looked almost everywhere. He decided to look in your house. He opened the door, and what she saw shocked him. His girlfriend on the couch with her best friend, kissing her.

He didn't even think Michonne liked girls, romantically or sexually. He was confused more than anything.