I had the same issues back in 2007. Went to get it investigated and found out that I have issues with my kidneys.
Had it not been for the high blood pressure and going for my class 1 I would have never of found out.
Sadly investigations are still on going and have now developed kidney stones which are being dealt with by the NHS.
As previously stated high blood pressure in a younger person is something to get investigated. I'm 32 and on blood pressure tablets.