What you need to do is see a Gastro Doc and get a colonoscopy. You might have Crohns, Ulcerative Colitis, Hemorrhoids (like ssc9 suggested), etc...
When I was 18 I was diagnosed with Ulcerative Colitis. It started with blood in the stool and then escalated into horrible stomach pain w/lots of blood in stool. I couldn't drink anything with caffeine or alcohol. I would be buckled over in the worse pain ever if I drank one or the other.
Please get checked out as soon as you can.
Good luck!
Lizzy
I recommend getting it checked out. Talk to your doctor about it. I know you're only 18, but it's better to get it checked out and have peace of mind then to wait until it gets worse (if it does get worse). It could just be hemorrhoids, but bright red stool may be even something like colon cancer (cancer doesn't care about age). I don't know your family history nor any other symptoms besides constipation (add more fiber in your diet for that...try drinking a glass of unsweetened prune juice morning and night and eat more fruit) so I'm just going on what I know. I hope this helps.