I sometimes have a little bright red blood on toilet paper after hard stool. what is the most likely cause?

Mild trauma: When stool is quite hard and typically difficult to pass, micro trauma occurs in the anal tissues. This can lead to an actual slit-type opening called a fissure and/or hemorrhoids. The fact that the blood is bright red indicates that it is coming from low down in the GI tract, i.e. anus. Work to soften stool: drink a generous amount of water, increase fiber in diet, daily exercise. Good wishes:)