Why would she go quiet when she thought I had left?

According to my best mate he said she said " oh no!" and then she paused and then said " not (my name) as well". This happened when she thought I had left. She then just stared at the floor for a solid minute. Until my mate confirmed it wasn't me that left and she cheered up again as a sign of relief.
Why would she go quiet when she thought I had left?
Post Opinion