a small fib at work

i was talking to my colleague jen today at the water cooler and she mentioned that our boss is looking for someone to stay late for a bit of overtime this week, but i already have plans with my grandkids on those evenings. so instead of telling her honestly that it’s not possible for me, i told her that i’d be interested in helping out, just so that i wouldn’t feel like the only one who couldn’t stick around to help out, you know? i mean, it’s not a big deal and she probably won’t even remember by next week when our boss makes their decision. but then after talking with jen, i started thinking about why i felt compelled to tell her that little fib, just so i wouldn’t be the one who can't do overtime for once in a while. is it because i’m trying to keep up appearances? am i scared of looking bad or like less of an employee than everyone else even though the truth has nothing to do with work at all but my personal life choices? or maybe it’s just because i don’t want to feel like a burden by being someone who can never be flexible for one small thing like this. but then again, why should that matter if i’m clear about the reasons why? why does feeling bad about others judging me even though i know they shouldn’t come before my actual commitments? it’s not like anyone really cares, and i certainly don’t have to lie just to avoid looking like a slacker or someone who always tries to get out of things. but still, here i am wondering if that small fib was worth the guilt i’m feeling now about being honest in the first place.