A line runs through my stools??!!

Hello,

I was wondering if someone can help. I have no bleeding or anything coming from my back passage when I pass stools, however, from time to time I notice a line (gap) runnning through my stool from top to bottom. Its about 2 - 3mm wide. It doesn't happen all the time but it looks strange.

A doctor gave me a rectal examination and said everything is perfect however I was wondering if an internal haemhorroid or my prostate is obstructing the passage of my stool and as a result causing this line?

Hope someone can help!