A.L.
If it's bright red, it's probably an anal fissure - a tiny little tear in the dry skin down there. If the blood were coming from inside, it would likely be dark.
Try putting a bit of vaseline or lanolin around her anus - sorry about the analogy, but sort of like you would with chapstick for a split lip. Same problem, different end. Moisturizing will help her be more comfortable, and help it heal faster.
And as Jo said, hard stools are a culprit, too.