What causes Rectal bleeding during a bowel movement?

Yes

High levels of both soluble and insoluble fibre.

Runny foods and water are mainstays of our diet when we are ill. Imbalances in diet, along w/ bacterial or viral infections panic our immune systems into action. Elimination of loose formless stools...

Possibly you ate something that had a lot of green it like spinach or salad greens.

Yes. Coffee contains a lot of caffeine (depending on brand and brewing method). This causes the intestines to become stimulated, and increased movement of the intestines causes a bowel movement.