Sometimes I feel sad for no reason, what could be the reason?

For instance, I traveled to a city last week for business meetings. Everything went well, and I had some good dinners with my colleagues and clients.
But there was a few times when I was in the room, lying on bed, I suddenly felt so sad and depressed, and I tried to watch something on YouTube or Netflix but it didn’t help at all.

Is this a psychological issue or it’s just something bad memories deeply in my mind but I can hardly realise?