Fiber supplement and urination

[Onigiriman]

I am 53, of East Asian descent, moderately active but a few pounds overweight. I occasionally take a fiber supplement to help with my digestive tract for mild constipation. Recently, I began taking the water-soluble fiber--store brand version (CVS) of Metamucil--once a day for four consecutive days. On the fifth day, I experienced pain (slight burning sensation) when I urinated. I also felt the need to urinate frequently, although the quantity was less than normal. I stopped taking the fiber and the pain subsided the next day and was gone by the third day. I had a hard time believing that fiber would effect painful urination, and so I took the fiber again, and the very next day I again experienced the same pain. Is there a relationship between water soluble fiber and urination? Can fiber enter the urinary system and "irritate" the urethra as it is being expelled? I have never had this problem when I took chewable tablet forms of fiber supplements. Any answer would be greatly appreciated.