Thank You Very Much Setaka Great Help :)
It sounds like you've already found the issue....
Your body systems cannot properly function if you don't. So being lethargic means your intestines aren't moving foods "as readily" as they should.
Stay with the water! That's great!
Being 16 you probably fast food yourself continually. Realize that bread, cheese, pizza, hambergers... things like that are giant "plugs" inside your intestines. They bind you up from going regularly.
Stay with the water, fruits and veggies if you want your system to be more "normal". And get away from the computer and take a walk. Eliminate the bikes, skateboards, etc and use the legs God gave you. That will help your body process food and move it through the intestine.
Just some ideas!