Would A Lump At The End Of The Rectum Be The Cause Of Colon Cancer?

Definitely have that checked out.  I'd even recommend seeing a colon and rectal surgeon as opposed to your family doctor.  You say lump, so I'm assuming it's a collection of something inside your body.  Not to scare you, but it may be an abscess or fistula which is painful to the touch.  Fistulas are somewhat common in Crohn's disease.  Don't ignore it this lump.