Problem With Accutane?

I had the problem of urinating frequently while taking accutane in 2010. I went to the doctor and got a blood test and a urine test and everything was normal. He told me to drink a lot of water and I did and it went away. I got the same problem in 2011 even after stopping accutane in 2010.Now I have it again. I am 16 now. Since the bood test and urine test is normal he told me to drink a lot of water.

Please help me.