Yep, lot's of possibilities. The most common "serious" causes are things like colitis, diverticulitis and Crohn's Disease. Less common causes would be cancer (though you'll think it's a common symptom if you google it). Go see your doctor and he'll undoubtedly have a colonoscopy performed to determine its cause.
Polyps more than likely. Maybe Hemorrhoids. I have both but have no pain or itching with either. Get some medicated wipes for hemorrhoids and see if that doesn't help. If not then go to the doc. At any rate.. Make sure your tushy really well, that will help too.
If it's bright red at the opening you probably have a cut or a hemrrhoid. No need to worry, It's common and needs to heal up. It can get out of hand though, like mine right now. So get some suppositories and Tucks. Good luck!