White Lie To The Boss

So at work today, my boss asked me how long it would take to finish that report. Knowing full well I couldn't do it by her deadline without staying overtime, I just said 'Oh no worries, should be fine' and walked off with a smile. Pretty sure she bought it too... But here's the thing: why am I feeling guilty about this? It wasn't like anyone got hurt or anything; I'll get it done eventually anyway.