What was Beckett doing when 城堡 told her it would be his last case? In 2x24 A Deadly Game

I Would say she was regretting those "This would be our Last Case" Word from Castle. Maybe she never thought he'd leave. Then after she realized that 城堡 cared for her, only then did she finally see how much she cared for him, hence the Looking at the empty chair at night. Waiting for him in the Morning.

She was shocked. I think she didn't know why until Esposito told her.