Is it normal to feel low for a few days without a reason?

Yes. Mood varies with sleep, illness, hormonal cycles, light exposure, blood sugar and countless things that never surface as a reason. A few low days that lift on their own are within normal range and generally do not need explaining. What warrants attention is persistence and function: low mood most of the day, most days, for two weeks or more, along with loss of interest in things you normally enjoy, changes in sleep or appetite, or difficulty doing ordinary tasks. The threshold is not how bad it feels but how long it lasts and what it stops you doing.