If you bleed in most of your digestive tract you digest the blood, all that remains is the iron which gives you black stools. If your red is blood it is coming from the last bit of your digestive system so could be haemmeroids.
If by red bits you mean blood clots then it may be anything from haemorrhoids, infective, inflammatory bowel or the big C. If any change in bowel habit, weight loss, abdominal pain then you should see your GP now.