What Causes Pain In The Middle Section Of Penis While Urinating?

Once in a while i feel a sharp pain while urinating not on the tip of my penis but somewhere in the middle and it is painful to the touch in that area. I have had bright red blood slowly come out for a short amount of time after urinating when this happens. It feels like something is catching maybe a sharp stone. i have no discharge at any other time nor any symptoms. what could be the cause?