I went through a stage like this too, I was constantly feeling sick and tired. It's likely that it is because you aren't eating well, just try and get alot of fruit and vegetables and high energy foods into your diet gradually and see if you start feeling better? I started seeing a dietician to try and keep me on track and that was good but not really necessary. If you feel tired but are getting the amount of sleep your body needs then it could be something like depression as Fuzzy suggested. Maybe just try one thing at a time and see if it helps (like more sleep, eating healthier foods, doing more exercise). Then you just stick with the combination of things that works!
And drink lots and lots of water!