After 2 weeks of checking everything in every way possible, they figured I had auto-immune hepatitis (body attacks my liver). That explained why I had been very tired, doctors said I'd take about 6 months before I could return to school, because I would be too tired and might get ill because my immune system would be weaker as I ate medicine limiting my immune system.
After 2 weeks of staying mostly home, eating immuno-repressant, I was bored as hell, went back to school and was back in full form, and I rarely get a cold or anything.
Never had any symptoms other than yellow eyes when it was worst, and extreme tiredness when it was the worst, apart from that it only shows itself when people actually analyze my blood...
No one really knows what the cause is, but there are many theories. It's fun to add some mystery to life, there's still a lot to discover ^^