Im 17 years old and i have a problem that i cant seem to work out.

In early July, i went to Ibiza and ate 3 good meals a day. But towards the end of the holiday, i suddenly had diarrhoea and felt nausea and ill. I was fine a few days later but suddenly fell ill again. For the past month its been on and off where i have bad stomachs. Then a few days ago, i seemed to have all the symptoms of a seasonal flu. But since then, ive felt weak, fatigued and i get full after eating several bites of a meal.

Im convinced my weight is the problem. I am 5 foot 8 and i weigh 7 stone 6 (104 lbs) and that is considerably underweight. Its possible what im feeling is because of the low weight i am or because my immune system couldnt fight off a virus from Ibiza due to my weight.

I've also checked the symptoms of diabetes and i have one or two symptoms which can be mistaken for anything (aching muscles, feeling ill after eating, fatigue). So basically i have a list of problems and i cant determine which one i have.

I dont have increased thirst.

My appetite is poor and i can only eat one meal a day and not feel hungry till the next day.

I have headaches which seem to bring on a bad stomach in some cases.

I dont always feel ill after eating.

I drink water above all.

I have days when i feel great (until recently) and then the next im ill again.

Im not generally fit and dont exercise (im always sat down or lying down most days).

Im alright in myself, as in a can still appear cheerful and fine.

So does anyone else know what the problem could be before i see a doctor?

Thanks in advance :)