Blockage in colon or rectum?

just a question is it possible to have a blockage in ur colon or rectum that could prevent bowels from ping i want to know because when i go to the bathroom the first stool comes out fine but after that i find myself pushing as hard as i can to get it out but nothing comes out and i lose the urge to go. however when i stand up i get the urge once again and i quickly sit down to p the bowels but i still have to push hard and when i do like hardly anything comes out(a little pebble usually) i also have sharp lower left abdomen pains that sometimes radiate to the right side and to the upper abdomen(sometimes) could this be due to a blockage or tumor in the colon or rectum or maybe possibly cancer i am an 18 year old male