I'm 24 and have had weird rib pressure that has me thinking colon cancer too, but I have a history of anxiety so I've been neglecting going to the doctor. It feels like this weird, tight/crampy feeling, but it does go away when I lay down. I think it's muscular and probably anxiety-induced ibs.
I've had it for about a year now, though I did have periods where it did not bother me. It's on the left side near the bottom.