It's okay and normal to just feel a bit sad or down sometimes.
Especially when you are in your teenage years, your brain can be experiencing many random emotions that don't seem to have real causes or make sense.
It could be because you are tired or stressed. Most people have limits to how much they can deal with or how much they can do in daily life before they get tired and get upset.
If it happens to you a lot, you may need to try more than usual to make yourself feel better.
The most important things to do are:
Try and have a good diet, nutrition has a very large influence on your mental health.
Exercise - you don't have to be a big sports dude, but a bit of walking or light exercise can release chemicals in your brain that make you feel better. Even if you are just feeling a bit down for a minute, a little quick walk can help.
And to try and do things which make you happy, it can be talking to friends or reading or drawing or just playing Pok?mon.
At the end of the day, anything you ever think or feel, is just activity in your brain. You can feel better if you do things to alter how your brain is behaving. Try not to worry too much about it, because everybody feels bad randomly sometimes.