I think I could have diabetes

I may be over-reacting but I am constantly thirsty and drink on average about 10 litres of fluid a day - this is not an exaggeration. I then have to go to the toilet soooooo many times a day. I have really blurred vision - but it is nothing to do with bad eyesight - as I went to the opticians and I am only very slightly short sightes. Also I am mostly tired and sometimes dizzy.
One day last week I went on a trip with my school and I didn't drink as much water as usual so that I wouldn't constanly be going to the toilet and I fainted on the coach. - this was in the morning. I have only ever fainted once before in my life - when I was 3. Do you think that this could be diabetes?