Why do I need to urinate so often?

In the morning I didn't have to go to the toilet until after 3 hours which was a record probably. But earlier I went to the toilet and half an hour I needed to go again. I did drink half a pint of squash though. So after that I didn't drink and within about half an hour again I needed to go, but not so much, and I have still been able to put it off like an extra hour more. I am worried because I have exams soon which are 2-3 hours long and I don't want to waste time leaving for the loo. My knickers also seem to be wet most of the time, and I wear pantyliners but the wetness soaks through.. Pelvic floor exercises haven't worked, been doing them for months..