The Small Lie

I recently told my girlfriend I had to attend an early morning professional development workshop at school when in reality, I was running late because my alarm didn’t go off. She doesn't know about it yet, but she did call me multiple times until we got home that night, making me feel like a fool for oversleeping. It’s petty and silly, but the guilt of not being honest has weighed on me more than I expected it to, even though this was just one instance out of a generally healthy relationship. Why is something so trivial becoming such a big deal in my mind? I know I should have been upfront about the situation since I'm used to telling her everything, but now I can't shake off the feeling of having betrayed her trust over nothing substantial.