You were acting off, he knew well. It wasn't that time of the month, he knew that too. So when he came into the bunker and saw you baking in the kitchen:

Dean absolutely knew something was off.

"Special occasion, babe?" He asked, putting his jacket onto the hook before he took a few steps closer and slid his hands under your shirt, thumbs running over your stomach. "What's got you in a bakin' mood, huh?"

There weren't any hunting get togethers, so you were up to something.