Depression can, and certainly does affect teenagers, regardless of what most people think. It can be chalked up to "just hormones", and sometimes it can be, but if you honestly feel otherwise, it could be clinical depression.
The most important thing, first of all: talk to someone. It may be embarrassing, but talk to a guidance counsellor at your school, or a doctor or someone in that sort of position. It may seem very intimidating, but they have the training to help you and they will do everything they can.
Depression can be triggered by many things, and sometimes it feels like absolutely nothing. But just feeling like this doesn't automatically mean that it's clinical depression. Talk to someone, for sure.
How often do you feel this? For how long at a time?