A bad mood explains your face. It doesn’t excuse your behavior.

A state and a decision are not the same thing. Feeling low accounts for your quietness and the flatness in your voice, and nobody is owed an apology for either, since moods cannot be manufactured on request. What it does not cover is the short answer you gave someone who asked a fair question. That part was chosen, even if it was chosen fast.

When the day is bad, say so early and briefly instead of letting people guess. Keep the basic courtesies running, since they ask very little of you. If you snap at somebody, mend it the same day while it is still small.