I am 24 years old, and I started having blood in my stool when I was 16... Bright red and it will last for weeks at a time. For years, I was told that it must be hemorrhoids and it was nothing to worry about.
I finally went to a gastroenterologist who did a colonoscopy on me. NO hemorrhoids.. But, I did have a small tear in my colon.
In more recent months, I have had the bright red blood mixed with stool that looks like mucus. I wonder if it is related to SLE. I go back to see a gastroenterologist in a couple of months to get another colonoscopy.
Don't always assume it is this or that. Go get it checked out. And if one doc ignores you, go onto the next. Someone will find the answer :)