Sometimes i'm thick sometimes i'm thin, but shape and size comes from within.

Most times i'm hard but sometimes not, i make you run clinching your butt.

While wearing pants and underware, last thing you want is see me there.

I am within you night and day, no matter if you pray or play.

To all the creatures i relate, and im the reason you are late.

Most often i go swim in bowl, with golden cornflakes it i troll,

 when flushing once will have its toll.

Sometimes i'm light and sometimes bleak, sometimes i smell and sometimes stink.

I'm seldom found in a sink.

What am I ?