2 of the simpler causes are diverticulosis
/itis (pouches in the colon
that get inflamed by seeds/nuts/popcorn & bleed) and hemorrhoids
(swollen rectal veins that may bleed or clot & be very painful).
A tear in the ring muscle of the anus (fissure) can also bleed heavily. Then we come to tumors: anal or rectal tumors or colon tumors (benign polyps or cancers) see doctor for thorough exam ± colonosc.