Blood or not???

I thought I saw a streak of blood in my stool this weekend.  I called the ER and asked if I should bring in a sample for testing.  They said not to but to come in for a rectal exam.  They did an exam and did a Hemoccult test that came back negative.  Does that mean it wasn't blood I saw in my stool about an hour earlier or could it have been blood that didn't leave any trace near the rectum after it passed?  I do have diverticulosis and the ER Doc said diverticulosis is the leading cause of passing blood.  I thought blood was rare with diverticulosis.  What can you tell me about that?  My overwhelming fear, of course, is colon cancer.  I'm 42, male and in constant anxiety about my health.  Any answers or assurances would be greatly appreciated.  Thanks.