First, get a good physical exam to be sure there is nothing "medically" wrong.
(also have to ask, are you doing anything or not doing something you have been doing--like using any drugs/alcohol etc?)
18 is a tough age...if you are still at home, do you have plans to move out? Are there changes you want to make but also fear?
Are things from the past returning?
Take care of yourself. You will get through this.