It could be depression. My depression started in my teens. Depression can come from external circumstances, things going on in your life. Or it can be internal and some crossed wires in your brain.
If it's external, perhaps the death of a loved one, taking healthy steps to deal with the grief (or whatever emotions apply to the situation) may help shorten the time you're affected. My father went through this and it lasted about a year before going away.
If it's internal, depending on severity, seeing a counselor could be a good place to start. They can make an official diagnosis and help you figure out the cause.
In either case being on a regular sleep schedule, eating a decent diet and getting exercise can help reduce depression.
If you have any more questions feel free to ask.